emacs loops when setting debug-on-error to t

[robert marshall]

I get emacs going into a loop using 100%
cpu when I set debug-on-error to t

At this point I have in the debug window

Debug-error: lisp error (void-function nil)
(nil)

but then can't interrupt it with Ctrl-g

I can replicate this by loading the vm mail reader (no local config),
loading a mail folder and then setting debug-on-error

In GNU Emacs 21.3.50.4 (i686-pc-linux-gnu, GTK+ Version 2.2.4)
 of 2004-05-17 on jailhouse.chezmarshall.freeserve.co.uk
configured using `configure '--with-x-toolkit=gtk''

Important settings:
  value of $LC_ALL: address@hidden
  value of $LC_COLLATE: en_GB
  value of $LC_CTYPE: en_GB
  value of $LC_MESSAGES: en_GB
  value of $LC_MONETARY: en_GB
  value of $LC_NUMERIC: en_GB
  value of $LC_TIME: en_GB
  value of $LANG: en_GB
  locale-coding-system: iso-latin-9
  default-enable-multibyte-characters: t

with gdb I get

Program received signal SIGTSTP, Stopped (user).
0x40677d71 in kill () from /lib/i686/libc.so.6
(gdb) where
#0  0x40677d71 in kill () from /lib/i686/libc.so.6
#1  0x081b85fa in wait_reading_process_input (time_limit=30, microsecs=0, 
    read_kbd=1, do_display=1) at process.c:4470
#2  0x08089ff1 in sit_for (sec=30, usec=0, reading=1, display=1, 
    initial_display=0) at dispnew.c:6407
#3  0x08124396 in read_char (commandflag=1, nmaps=2, maps=0xbfffdb40, 
    prev_event=138481465, used_mouse_menu=0xbfffdb78) at keyboard.c:2697
#4  0x0812afe1 in read_key_sequence (keybuf=0xbfffdca0, bufsize=30, 
    prompt=138481465, dont_downcase_last=0, can_return_switch_frame=1, 
    fix_current_buffer=1) at keyboard.c:8770
#5  0x08121077 in command_loop_1 () at keyboard.c:1476
#6  0x081823bd in internal_condition_case (bfun=0x8120ed0 <command_loop_1>, 
    handlers=138542401, hfun=0x8120a50 <cmd_error>) at eval.c:1333
#7  0x08120d5b in command_loop_2 () at keyboard.c:1264
#8  0x08181eda in internal_catch (tag=0, func=0x8120d40 <command_loop_2>, 
    arg=138481465) at eval.c:1094
#9  0x08120cc7 in command_loop () at keyboard.c:1235
#10 0x081207b2 in recursive_edit_1 () at keyboard.c:959
#11 0x08120900 in Frecursive_edit () at keyboard.c:1015
#12 0x081840d4 in Ffuncall (nargs=1, args=0x3d) at eval.c:2723