> Can you switch to that buffer and look what's at point?  And while at
 > that, post here the full contents of that buffer?

It just happened again while debugging xdisp.c.  The buffer is called
*partial-output-emacs.exe* (with a leading space) it contains 2295
characters and its position of point was 2.  The contents of that buffer
are hopefully in the attached file partial-output-emacs.txt (for reasons
that elude me I was not able to inline them with Thunderbird).

The contents of *backtrace* are:

Debugger entered--Lisp error: (json-string-format "doesn't start with `\"'!")
   signal(json-string-format ("doesn't start with `\"'!"))
   (if (char-equal (json-peek) 34) nil (message "%s ... %s" (current-buffer) (point)) (signal (quote json-string-format) (list "doesn't start with `\"'!")))
   json-read-string()
   json-read-object()
   apply(json-read-object nil)
   json-read()
   gdb-json-read-buffer(nil nil)
   gdb-json-partial-output()
   gdb-thread-list-handler-custom()
   gdb-thread-list-handler()
   apply(gdb-thread-list-handler nil)
   (save-current-buffer (set-buffer #<buffer *threads of emacs.exe*>) (apply (quote gdb-thread-list-handler) args))
   (with-current-buffer #<buffer *threads of emacs.exe*> (apply (quote gdb-thread-list-handler) args))
   (lambda (&rest args) (with-current-buffer #<buffer *threads of emacs.exe*> (apply (quote gdb-thread-list-handler) args)))()
   gdb-handle-reply(57)
   gdb-done-or-error("57" done "threads=[{id=\"3\",target-id=\"Thread 2608.0xa60\",frame={level=\"0\",addr=\"0x7c91eb94\",func=\"ntdll!LdrAccessResource\",args=[],from=\"C:\\\\WINDOWS\\\\system32\\\\ntdll.dll\"},state=\"stopped\"},{id=\"2\",target-id=\"Thread 2608.0xa64\",frame={level=\"0\",addr=\"0x7c91eb94\",func=\"ntdll!LdrAccessResource\",args=[],from=\"C:\\\\WINDOWS\\\\system32\\\\ntdll.dll\"},state=\"stopped\"},{id=\"1\",target-id=\"Thread 2608.0xb0c\",frame={level=\"0\",addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],file=\"../../src/xdisp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},state=\"stopped\"}],current-thread-id=\"1\"" t)
   gdb-done("57" "threads=[{id=\"3\",target-id=\"Thread 2608.0xa60\",frame={level=\"0\",addr=\"0x7c91eb94\",func=\"ntdll!LdrAccessResource\",args=[],from=\"C:\\\\WINDOWS\\\\system32\\\\ntdll.dll\"},state=\"stopped\"},{id=\"2\",target-id=\"Thread 2608.0xa64\",frame={level=\"0\",addr=\"0x7c91eb94\",func=\"ntdll!LdrAccessResource\",args=[],from=\"C:\\\\WINDOWS\\\\system32\\\\ntdll.dll\"},state=\"stopped\"},{id=\"1\",target-id=\"Thread 2608.0xb0c\",frame={level=\"0\",addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],file=\"../../src/xdisp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},state=\"stopped\"}],current-thread-id=\"1\"" t)
   gdbmi-bnf-incomplete-record-result("57" (gdb-done . progressive))
   #[0 "\302\300\301\242\"\207" ["57" ((gdb-done . progressive)) gdbmi-bnf-incomplete-record-result] 3]()
   gdbmi-bnf-result-and-async-record-impl()
   gdbmi-bnf-async-record()
   gdbmi-bnf-out-of-band-record()
   gdbmi-bnf-output()
   gud-gdbmi-marker-filter("~\"Continuing.\\n\"\n^running\n*running,thread-id=\"1\"\n(gdb) \n*running,thread-id=\"all\"\n&\"warning: frame 01b50060 (emacs@MACHNO) obscured\\n\"\n&\"\\n\"\n=breakpoint-modified,bkpt={number=\"3\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0105ee62\",func=\"redisplay_internal\",file=\"../../src/xdisp.c\",fullname=\"c:\\\\\\\\emacs-git\\\\\\\\trunk\\\\\\\\src\\\\\\\\xdisp.c\",line=\"13512\",times=\"13\",original-location=\"xdisp.c:13512\"}\n~\"\\nBreakpoint \"\n~\"3, redisplay_internal () at ../../src/xdisp.c:13512\\n\"\n~\"13512\\t  if ((!NILP (echo_area_buffer[0]) && !display_last_displayed_message_p)\\n\"\n*stopped,reason=\"breakpoint-hit\",disp=\"keep\",bkptno=\"3\",frame={addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],file=\"../../s
 rc/xdisp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},thread-id=\"1\",stopped-threads=\"all\"\n(gdb) \n")
   apply(gud-gdbmi-marker-filter "~\"Continuing.\\n\"\n^running\n*running,thread-id=\"1\"\n(gdb) \n*running,thread-id=\"all\"\n&\"warning: frame 01b50060 (emacs@MACHNO) obscured\\n\"\n&\"\\n\"\n=breakpoint-modified,bkpt={number=\"3\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0105ee62\",func=\"redisplay_internal\",file=\"../../src/xdisp.c\",fullname=\"c:\\\\\\\\emacs-git\\\\\\\\trunk\\\\\\\\src\\\\\\\\xdisp.c\",line=\"13512\",times=\"13\",original-location=\"xdisp.c:13512\"}\n~\"\\nBreakpoint \"\n~\"3, redisplay_internal () at ../../src/xdisp.c:13512\\n\"\n~\"13512\\t  if ((!NILP (echo_area_buffer[0]) && !display_last_displayed_message_p)\\n\"\n*stopped,reason=\"breakpoint-hit\",disp=\"keep\",bkptno=\"3\",frame={addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],file=\".
 ./../src/xdisp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},thread-id=\"1\",stopped-threads=\"all\"\n(gdb) \n")
   gud-marker-filter("~\"Continuing.\\n\"\n^running\n*running,thread-id=\"1\"\n(gdb) \n*running,thread-id=\"all\"\n&\"warning: frame 01b50060 (emacs@MACHNO) obscured\\n\"\n&\"\\n\"\n=breakpoint-modified,bkpt={number=\"3\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0105ee62\",func=\"redisplay_internal\",file=\"../../src/xdisp.c\",fullname=\"c:\\\\\\\\emacs-git\\\\\\\\trunk\\\\\\\\src\\\\\\\\xdisp.c\",line=\"13512\",times=\"13\",original-location=\"xdisp.c:13512\"}\n~\"\\nBreakpoint \"\n~\"3, redisplay_internal () at ../../src/xdisp.c:13512\\n\"\n~\"13512\\t  if ((!NILP (echo_area_buffer[0]) && !display_last_displayed_message_p)\\n\"\n*stopped,reason=\"breakpoint-hit\",disp=\"keep\",bkptno=\"3\",frame={addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],file=\"../../src/xdi
 sp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},thread-id=\"1\",stopped-threads=\"all\"\n(gdb) \n")
   gud-filter(#<process gud-emacs.exe> "~\"Continuing.\\n\"\n^running\n*running,thread-id=\"1\"\n(gdb) \n*running,thread-id=\"all\"\n&\"warning: frame 01b50060 (emacs@MACHNO) obscured\\n\"\n&\"\\n\"\n=breakpoint-modified,bkpt={number=\"3\",type=\"breakpoint\",disp=\"keep\",enabled=\"y\",addr=\"0x0105ee62\",func=\"redisplay_internal\",file=\"../../src/xdisp.c\",fullname=\"c:\\\\\\\\emacs-git\\\\\\\\trunk\\\\\\\\src\\\\\\\\xdisp.c\",line=\"13512\",times=\"13\",original-location=\"xdisp.c:13512\"}\n~\"\\nBreakpoint \"\n~\"3, redisplay_internal () at ../../src/xdisp.c:13512\\n\"\n~\"13512\\t  if ((!NILP (echo_area_buffer[0]) && !display_last_displayed_message_p)\\n\"\n*stopped,reason=\"breakpoint-hit\",disp=\"keep\",bkptno=\"3\",frame={addr=\"0x0105ee62\",func=\"redisplay_internal\",args=[],fi
 le=\"../../src/xdisp.c\",fullname=\"c:\\\\emacs-git\\\\trunk\\\\src\\\\xdisp.c\",line=\"13512\"},thread-id=\"1\",stopped-threads=\"all\"\n(gdb) \n")

martin