diff --git a/console.c b/console.c
index 62c64aff..78c2da1b 100644
--- a/console.c
+++ b/console.c
@@ -79,6 +79,11 @@ void console_connection_fatal(Seat *seat, const char *msg)
     cleanup_exit(1);
 }
 
+void console_nonfatal(Seat *seat, const char *msg)
+{
+    console_print_error_msg("ERROR", msg);
+}
+
 /*
  * Console front ends redo their select() or equivalent every time, so
  * they don't need separate timer handling.
diff --git a/proxy/sshproxy.c b/proxy/sshproxy.c
index 165c6c0a..8a902e08 100644
--- a/proxy/sshproxy.c
+++ b/proxy/sshproxy.c
@@ -405,6 +405,14 @@ static void sshproxy_connection_fatal(Seat *seat, const char *message)
     }
 }
 
+static void sshproxy_nonfatal(Seat *seat, const char *message)
+{
+    SshProxy *sp = container_of(seat, SshProxy, seat);
+    if (sp->clientseat)
+        seat_nonfatal(sp->clientseat, "error in proxy SSH connection: %s",
+                      message);
+}
+
 static SeatPromptResult sshproxy_confirm_ssh_host_key(
     Seat *seat, const char *host, int port, const char *keytype,
     char *keystr, SeatDialogText *text, HelpCtx helpctx,
@@ -541,6 +549,7 @@ static const SeatVtable SshProxy_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = sshproxy_notify_remote_disconnect,
     .connection_fatal = sshproxy_connection_fatal,
+    .nonfatal = sshproxy_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/pscp.c b/pscp.c
index 77de1cd9..87fb1722 100644
--- a/pscp.c
+++ b/pscp.c
@@ -71,6 +71,7 @@ static const SeatVtable pscp_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
+    .nonfatal = console_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/psftp.c b/psftp.c
index d8b5c400..bf8782d3 100644
--- a/psftp.c
+++ b/psftp.c
@@ -52,6 +52,7 @@ static const SeatVtable psftp_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
+    .nonfatal = console_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/psocks.c b/psocks.c
index eb7c8f01..f9e76949 100644
--- a/psocks.c
+++ b/psocks.c
@@ -529,7 +529,7 @@ int check_stored_host_key(const char *hostname, int port,
     unreachable("host keys not handled in this tool");
 }
 
-void store_host_key(const char *hostname, int port,
+void store_host_key(Seat *seat, const char *hostname, int port,
                     const char *keytype, const char *key)
 {
     unreachable("host keys not handled in this tool");
diff --git a/putty.h b/putty.h
index 5c3adfe9..9eb6c0b5 100644
--- a/putty.h
+++ b/putty.h
@@ -1214,9 +1214,15 @@ struct SeatVtable {
     void (*notify_remote_disconnect)(Seat *seat);
 
     /*
-     * Notify the seat that the connection has suffered a fatal error.
+     * Notify the seat that the connection has suffered an error,
+     * either fatal to the whole connection or not.
+     *
+     * The latter kind of error is expected to be things along the
+     * lines of 'I/O error storing the new host key', which has
+     * traditionally been presented via a dialog box or similar.
      */
     void (*connection_fatal)(Seat *seat, const char *message);
+    void (*nonfatal)(Seat *seat, const char *message);
 
     /*
      * Notify the seat that the list of special commands available
@@ -1481,10 +1487,11 @@ static inline bool seat_interactive(Seat *seat)
 static inline bool seat_get_cursor_position(Seat *seat, int *x, int *y)
 { return  seat->vt->get_cursor_position(seat, x, y); }
 
-/* Unlike the seat's actual method, the public entry point
- * seat_connection_fatal is a wrapper function with a printf-like API,
- * defined in utils. */
+/* Unlike the seat's actual method, the public entry points
+ * seat_connection_fatal and seat_nonfatal are wrapper functions with
+ * a printf-like API, defined in utils. */
 void seat_connection_fatal(Seat *seat, const char *fmt, ...) PRINTF_LIKE(2, 3);
+void seat_nonfatal(Seat *seat, const char *fmt, ...) PRINTF_LIKE(2, 3);
 
 /* Handy aliases for seat_output which set is_stderr to a fixed value. */
 static inline size_t seat_stdout(Seat *seat, const void *data, size_t len)
@@ -1528,6 +1535,7 @@ void nullseat_notify_session_started(Seat *seat);
 void nullseat_notify_remote_exit(Seat *seat);
 void nullseat_notify_remote_disconnect(Seat *seat);
 void nullseat_connection_fatal(Seat *seat, const char *message);
+void nullseat_nonfatal(Seat *seat, const char *message);
 void nullseat_update_specials_menu(Seat *seat);
 char *nullseat_get_ttymode(Seat *seat, const char *mode);
 void nullseat_set_busy_status(Seat *seat, BusyStatus status);
@@ -1567,6 +1575,7 @@ bool nullseat_get_cursor_position(Seat *seat, int *x, int *y);
  */
 
 void console_connection_fatal(Seat *seat, const char *message);
+void console_nonfatal(Seat *seat, const char *message);
 SeatPromptResult console_confirm_ssh_host_key(
     Seat *seat, const char *host, int port, const char *keytype,
     char *keystr, SeatDialogText *text, HelpCtx helpctx,
diff --git a/ssh/kex2-client.c b/ssh/kex2-client.c
index b890c023..8ca8cd40 100644
--- a/ssh/kex2-client.c
+++ b/ssh/kex2-client.c
@@ -1021,7 +1021,7 @@ void ssh2kex_coroutine(struct ssh2_transport_state *s, bool *aborted)
             ppl_logevent("%s", fingerprint);
             sfree(fingerprint);
 
-            store_host_key(s->savedhost, s->savedport,
+            store_host_key(s->ppl.seat, s->savedhost, s->savedport,
                            ssh_key_cache_id(s->hkey), s->keystr);
             /*
              * Don't forget to store the new key as the one we'll be
diff --git a/ssh/server.c b/ssh/server.c
index 188426b3..4e315874 100644
--- a/ssh/server.c
+++ b/ssh/server.c
@@ -116,6 +116,7 @@ static const SeatVtable server_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = nullseat_connection_fatal,
+    .nonfatal = nullseat_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/ssh/sesschan.c b/ssh/sesschan.c
index 9776eaf2..b500f77d 100644
--- a/ssh/sesschan.c
+++ b/ssh/sesschan.c
@@ -193,6 +193,7 @@ static const SeatVtable sesschan_seat_vt = {
     .notify_remote_exit = sesschan_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = sesschan_connection_fatal,
+    .nonfatal = nullseat_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/storage.h b/storage.h
index e9138f40..09e61368 100644
--- a/storage.h
+++ b/storage.h
@@ -89,8 +89,10 @@ int check_stored_host_key(const char *hostname, int port,
 /*
  * Write a host key into the database, overwriting any previous
  * entry that might have been there.
+ *
+ * A Seat is provided for error-reporting purposes.
  */
-void store_host_key(const char *hostname, int port,
+void store_host_key(Seat *seat, const char *hostname, int port,
                     const char *keytype, const char *key);
 
 /* ----------------------------------------------------------------------
diff --git a/stubs/null-seat.c b/stubs/null-seat.c
index 37cb0f4c..80d8b27f 100644
--- a/stubs/null-seat.c
+++ b/stubs/null-seat.c
@@ -17,6 +17,7 @@ void nullseat_notify_session_started(Seat *seat) {}
 void nullseat_notify_remote_exit(Seat *seat) {}
 void nullseat_notify_remote_disconnect(Seat *seat) {}
 void nullseat_connection_fatal(Seat *seat, const char *message) {}
+void nullseat_nonfatal(Seat *seat, const char *message) {}
 void nullseat_update_specials_menu(Seat *seat) {}
 char *nullseat_get_ttymode(Seat *seat, const char *mode) { return NULL; }
 void nullseat_set_busy_status(Seat *seat, BusyStatus status) {}
diff --git a/unix/console.c b/unix/console.c
index 286ecf29..495783b1 100644
--- a/unix/console.c
+++ b/unix/console.c
@@ -191,7 +191,7 @@ SeatPromptResult console_confirm_ssh_host_key(
     if (line[0] != '\0' && line[0] != '\r' && line[0] != '\n' &&
         line[0] != 'q' && line[0] != 'Q') {
         if (line[0] == 'y' || line[0] == 'Y')
-            store_host_key(host, port, keytype, keystr);
+            store_host_key(seat, host, port, keytype, keystr);
         postmsg(&cf);
         return SPR_OK;
     } else {
diff --git a/unix/dialog.c b/unix/dialog.c
index 0d9d9573..2ff51ede 100644
--- a/unix/dialog.c
+++ b/unix/dialog.c
@@ -3529,7 +3529,8 @@ static void confirm_ssh_host_key_result_callback(void *vctx, int result)
          * doesn't care whether we saved the host key or not).
          */
         if (result == 2) {
-            store_host_key(ctx->host, ctx->port, ctx->keytype, ctx->keystr);
+            store_host_key(ctx->seat, ctx->host, ctx->port,
+                           ctx->keytype, ctx->keystr);
             logical_result = SPR_OK;
         } else if (result == 1) {
             logical_result = SPR_OK;
diff --git a/unix/plink.c b/unix/plink.c
index f0c73754..c1738e10 100644
--- a/unix/plink.c
+++ b/unix/plink.c
@@ -409,6 +409,7 @@ static const SeatVtable plink_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
+    .nonfatal = console_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = plink_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/unix/storage.c b/unix/storage.c
index ca225732..35a276c2 100644
--- a/unix/storage.c
+++ b/unix/storage.c
@@ -828,7 +828,7 @@ bool have_ssh_host_key(const char *hostname, int port,
     return check_stored_host_key(hostname, port, keytype, "") != 1;
 }
 
-void store_host_key(const char *hostname, int port,
+void store_host_key(Seat *seat, const char *hostname, int port,
                     const char *keytype, const char *key)
 {
     FILE *rfp, *wfp;
@@ -846,7 +846,7 @@ void store_host_key(const char *hostname, int port,
 
         dir = make_filename(INDEX_DIR, NULL);
         if ((errmsg = make_dir_path(dir, 0700)) != NULL) {
-            nonfatal("Unable to store host key: %s", errmsg);
+            seat_nonfatal(seat, "Unable to store host key: %s", errmsg);
             sfree(errmsg);
             sfree(dir);
             sfree(tmpfilename);
@@ -857,8 +857,8 @@ void store_host_key(const char *hostname, int port,
         wfp = fopen(tmpfilename, "w");
     }
     if (!wfp) {
-        nonfatal("Unable to store host key: open(\"%s\") "
-                 "returned '%s'", tmpfilename, strerror(errno));
+        seat_nonfatal(seat, "Unable to store host key: open(\"%s\") "
+                      "returned '%s'", tmpfilename, strerror(errno));
         sfree(tmpfilename);
         return;
     }
@@ -889,9 +889,9 @@ void store_host_key(const char *hostname, int port,
     fclose(wfp);
 
     if (rename(tmpfilename, filename) < 0) {
-        nonfatal("Unable to store host key: rename(\"%s\",\"%s\")"
-                 " returned '%s'", tmpfilename, filename,
-                 strerror(errno));
+        seat_nonfatal(seat, "Unable to store host key: rename(\"%s\",\"%s\")"
+                      " returned '%s'", tmpfilename, filename,
+                      strerror(errno));
     }
 
     sfree(tmpfilename);
diff --git a/unix/window.c b/unix/window.c
index 1958edde..de75a1c2 100644
--- a/unix/window.c
+++ b/unix/window.c
@@ -290,6 +290,12 @@ static void gtk_seat_connection_fatal(Seat *seat, const char *msg)
     queue_toplevel_callback(connection_fatal_callback, inst);
 }
 
+static void gtk_seat_nonfatal(Seat *seat, const char *msg)
+{
+    GtkFrontend *inst = container_of(seat, GtkFrontend, seat);
+    nonfatal_message_box(inst->window, msg);
+}
+
 /*
  * Default settings that are specific to pterm.
  */
@@ -423,6 +429,7 @@ static const SeatVtable gtk_seat_vt = {
     .notify_remote_exit = gtk_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = gtk_seat_connection_fatal,
+    .nonfatal = gtk_seat_nonfatal,
     .update_specials_menu = gtk_seat_update_specials_menu,
     .get_ttymode = gtk_seat_get_ttymode,
     .set_busy_status = gtk_seat_set_busy_status,
diff --git a/utils/CMakeLists.txt b/utils/CMakeLists.txt
index 6ef33c8d..c09027f4 100644
--- a/utils/CMakeLists.txt
+++ b/utils/CMakeLists.txt
@@ -51,6 +51,7 @@ add_sources_from_current_dir(utils
   read_file_into.c
   seat_connection_fatal.c
   seat_dialog_text.c
+  seat_nonfatal.c
   sessprep.c
   sk_free_peer_info.c
   smemclr.c
diff --git a/utils/seat_nonfatal.c b/utils/seat_nonfatal.c
new file mode 100644
index 00000000..d21e4c17
--- /dev/null
+++ b/utils/seat_nonfatal.c
@@ -0,0 +1,19 @@
+/*
+ * Wrapper function for the nonfatal() method of a Seat,
+ * providing printf-style formatting.
+ */
+
+#include "putty.h"
+
+void seat_nonfatal(Seat *seat, const char *fmt, ...)
+{
+    va_list ap;
+    char *msg;
+
+    va_start(ap, fmt);
+    msg = dupvprintf(fmt, ap);
+    va_end(ap);
+
+    seat->vt->nonfatal(seat, msg);
+    sfree(msg);
+}
diff --git a/utils/tempseat.c b/utils/tempseat.c
index ad37b0e3..a8d99cc2 100644
--- a/utils/tempseat.c
+++ b/utils/tempseat.c
@@ -288,6 +288,17 @@ static void tempseat_connection_fatal(Seat *seat, const char *message)
     unreachable("connection_fatal should never be called on TempSeat");
 }
 
+static void tempseat_nonfatal(Seat *seat, const char *message)
+{
+    /*
+     * Non-fatal errors specific to a Seat should also not occur,
+     * because those will be for things like I/O errors writing the
+     * host key collection, and a backend's not _doing_ that when we
+     * haven't connected it to the host yet.
+     */
+    unreachable("nonfatal should never be called on TempSeat");
+}
+
 static bool tempseat_eof(Seat *seat)
 {
     /*
@@ -327,6 +338,7 @@ static const struct SeatVtable tempseat_vt = {
     .notify_remote_exit = tempseat_notify_remote_exit,
     .notify_remote_disconnect = tempseat_notify_remote_disconnect,
     .connection_fatal = tempseat_connection_fatal,
+    .nonfatal = tempseat_nonfatal,
     .update_specials_menu = tempseat_update_specials_menu,
     .get_ttymode = tempseat_get_ttymode,
     .set_busy_status = tempseat_set_busy_status,
diff --git a/windows/console.c b/windows/console.c
index 2fd572b4..b6ecb203 100644
--- a/windows/console.c
+++ b/windows/console.c
@@ -119,7 +119,7 @@ SeatPromptResult console_confirm_ssh_host_key(
     if (line[0] != '\0' && line[0] != '\r' && line[0] != '\n' &&
         line[0] != 'q' && line[0] != 'Q') {
         if (line[0] == 'y' || line[0] == 'Y')
-            store_host_key(host, port, keytype, keystr);
+            store_host_key(seat, host, port, keytype, keystr);
         return SPR_OK;
     } else {
         fputs(console_abandoned_msg, stderr);
diff --git a/windows/dialog.c b/windows/dialog.c
index c34703f6..68d88df8 100644
--- a/windows/dialog.c
+++ b/windows/dialog.c
@@ -1155,7 +1155,7 @@ SeatPromptResult win_seat_confirm_ssh_host_key(
         wgs->term_hwnd, HostKeyDialogProc, ctx);
     assert(mbret==IDC_HK_ACCEPT || mbret==IDC_HK_ONCE || mbret==IDCANCEL);
     if (mbret == IDC_HK_ACCEPT) {
-        store_host_key(host, port, keytype, keystr);
+        store_host_key(seat, host, port, keytype, keystr);
         return SPR_OK;
     } else if (mbret == IDC_HK_ONCE) {
         return SPR_OK;
diff --git a/windows/plink.c b/windows/plink.c
index 05c25073..48429ba2 100644
--- a/windows/plink.c
+++ b/windows/plink.c
@@ -97,6 +97,7 @@ static const SeatVtable plink_seat_vt = {
     .notify_remote_exit = nullseat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = console_connection_fatal,
+    .nonfatal = console_nonfatal,
     .update_specials_menu = nullseat_update_specials_menu,
     .get_ttymode = nullseat_get_ttymode,
     .set_busy_status = nullseat_set_busy_status,
diff --git a/windows/storage.c b/windows/storage.c
index f2c33feb..7736b3db 100644
--- a/windows/storage.c
+++ b/windows/storage.c
@@ -357,7 +357,7 @@ bool have_ssh_host_key(const char *hostname, int port,
     return check_stored_host_key(hostname, port, keytype, "") != 1;
 }
 
-void store_host_key(const char *hostname, int port,
+void store_host_key(Seat *seat, const char *hostname, int port,
                     const char *keytype, const char *key)
 {
     strbuf *regname = strbuf_new();
diff --git a/windows/window.c b/windows/window.c
index bd26e733..35236f2d 100644
--- a/windows/window.c
+++ b/windows/window.c
@@ -321,6 +321,7 @@ static bool win_seat_eof(Seat *seat);
 static SeatPromptResult win_seat_get_userpass_input(Seat *seat, prompts_t *p);
 static void win_seat_notify_remote_exit(Seat *seat);
 static void win_seat_connection_fatal(Seat *seat, const char *msg);
+static void win_seat_nonfatal(Seat *seat, const char *msg);
 static void win_seat_update_specials_menu(Seat *seat);
 static void win_seat_set_busy_status(Seat *seat, BusyStatus status);
 static void win_seat_set_trust_status(Seat *seat, bool trusted);
@@ -338,6 +339,7 @@ static const SeatVtable win_seat_vt = {
     .notify_remote_exit = win_seat_notify_remote_exit,
     .notify_remote_disconnect = nullseat_notify_remote_disconnect,
     .connection_fatal = win_seat_connection_fatal,
+    .nonfatal = win_seat_nonfatal,
     .update_specials_menu = win_seat_update_specials_menu,
     .get_ttymode = win_seat_get_ttymode,
     .set_busy_status = win_seat_set_busy_status,
@@ -1188,6 +1190,17 @@ static void win_seat_connection_fatal(Seat *seat, const char *msg)
     }
 }
 
+/*
+ * Print a message box and don't close the connection.
+ */
+static void win_seat_nonfatal(Seat *seat, const char *msg)
+{
+    char *title = dupprintf("%s Error", appname);
+    show_mouseptr(true);
+    MessageBox(wgs.term_hwnd, msg, title, MB_ICONERROR | MB_OK);
+    sfree(title);
+}
+
 /*
  * Report an error at the command-line parsing stage.
  */