To reproduce:

1. xemacs -vanilla
2. create a second frame (C-x 5 2)
3. position the frames so that the menubars of each are visible
4. make sure one of the frames has the input focus
5. click on Buffers in the menubar of the other frame

Lisp stack:
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (catch #<INTERNAL OBJECT (XEmacs bug?) (opaque, size=0) 0x0> ...)
  # (unwind-protect ...)
  # bind (inhibit-quit)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # bind (inhibit-quit)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  # (unwind-protect ...)
  ("(in redisplay)" "[internal]")
  # (unwind-protect ...)
  # bind (inhibit-quit)
  (next-event "[internal]")
  # (condition-case ... . error)
  # (catch top-level ...)

C stack:
#0  0x008a5416 in __kernel_vsyscall ()
#1  0x001f1c36 in kill () from /lib/libc.so.6
#2  0x080e4d36 in fatal_error_signal (sig=6) at emacs.c:3903
#3  <signal handler called>
#4  0x008a5416 in __kernel_vsyscall ()
#5  0x001f1941 in raise () from /lib/libc.so.6
#6  0x001f4e42 in abort () from /lib/libc.so.6
#7  0x080e61e9 in really_abort () at emacs.c:4792
#8  0x080e4e3c in assert_failed (file=0x830a98b "signal.c", line=733, 
    expr=0x830aa24 "QUIT called from within redisplay without being
properly wrapped") at emacs.c:4089
#9  0x0824c5e9 in check_quit () at signal.c:731
#10 0x0824c198 in check_what_happened () at signal.c:461
#11 0x080eeca4 in Ffuncall (nargs=2, args=0xbfb4e1e4) at eval.c:4054
#12 0x080f1ae6 in call1 (fn=137699776, arg0=138102076) at eval.c:5400
#13 0x082b05d3 in menu_item_descriptor_to_widget_value_1 (desc=138102076, 
    menu_type=1, deep_p=1, filter_p=0, depth=0) at menubar-x.c:216
#14 0x082b096b in protected_menu_item_descriptor_to_widget_value_1 (
    gack=0xbfb4e564) at menubar-x.c:351
#15 0x080f25c4 in call_trapping_problems_2 (opaque=146991152) at eval.c:5788
#16 0x080eb386 in call_with_condition_handler (
    handler=0x80f24fc <flagged_a_squirmer>, handler_arg=146991152, 
    fun=0x80f258c <call_trapping_problems_2>, arg=146991152) at eval.c:2356
#17 0x080f25ee in call_trapping_problems_1 (opaque=146991152) at eval.c:5794
#18 0x080ea5e5 in internal_catch (tag=139635344, 
    func=0x80f25c6 <call_trapping_problems_1>, arg=146991152, 
    threw=0xbfb4e4e8, thrown_tag=0xbfb4e4f4,
backtrace_before_throw=0xbfb4e4ec)
    at eval.c:1723
#19 0x080f29ff in call_trapping_problems (warning_class=137815768, 
    warning_string=0x8315e41 "Error during menu callback", flags=35, 
    problem=0x0, 
    fun=0x82b0923 <protected_menu_item_descriptor_to_widget_value_1>, 
    arg=0xbfb4e564) at eval.c:6067
#20 0x08103964 in event_stream_protect_modal_loop (
    error_string=0x8315e41 "Error during menu callback", 
    bfun=0x82b0923 <protected_menu_item_descriptor_to_widget_value_1>, 
    barg=0xbfb4e564, flags=32) at event-stream.c:1914
#21 0x082b09e8 in protected_menu_item_descriptor_to_widget_value (
    desc=138102112, menu_type=1, deep_p=1, filter_p=0) at menubar-x.c:380
#22 0x082b0b83 in pre_activate_callback (widget=0x9072bd0, unused_id=65544, 
    client_data=0x906f580) at menubar-x.c:474
#23 0x082d3626 in pre_hook (w=0x9072bd0, client_data=0x9072bb8, 
    call_data=0x906f580) at lwlib-Xlw.c:81
#24 0x0012f54c in XtCallCallbackList () from /usr/lib/libXt.so.6
#25 0x082ceaa1 in remap_menubar (mw=0x9072bd0) at xlwmenu.c:2714
#26 0x082d025c in handle_single_motion_event (mw=0x9072bd0, ev=0xbfb4ed1c, 
    select_p=1 '\001') at xlwmenu.c:3502
#27 0x082d0363 in handle_motion_event (mw=0x9072bd0, ev=0xbfb4ed1c, 
    select_p=1 '\001') at xlwmenu.c:3534
#28 0x082d04a6 in Start (w=0x9072bd0, ev=0xbfb4ed1c, unused_params=0x0, 
    unused_num_params=0x174df8) at xlwmenu.c:3572
#29 0x001674cb in ?? () from /usr/lib/libXt.so.6
#30 0x00167618 in ?? () from /usr/lib/libXt.so.6
#31 0x00167ecc in _XtTranslateEvent () from /usr/lib/libXt.so.6
#32 0x0013dd2a in XtDispatchEventToWidget () from /usr/lib/libXt.so.6
#33 0x0013e56b in ?? () from /usr/lib/libXt.so.6
#34 0x0013d3fd in XtDispatchEvent () from /usr/lib/libXt.so.6
#35 0x0014ae5d in XtAppProcessEvent () from /usr/lib/libXt.so.6
#36 0x0828fea0 in emacs_Xt_drain_queue () at event-Xt.c:2790
#37 0x0828ff46 in check_if_pending_expose_event (dev=0x8effd48)
    at event-Xt.c:2811
#38 0x082a9510 in x_window_output_end (w=0x8f1ed50) at redisplay-x.c:58
#39 0x0822f86e in redisplay_output_window (w=0x8f1ed50)
    at redisplay-output.c:2524
#40 0x0821c4a0 in redisplay_window (window=150072656, skip_selected=0)
    at redisplay.c:6585
#41 0x0821cf3e in redisplay_frame (f=0x8f1ebd8, preemption_check=1)
    at redisplay.c:7004
#42 0x0821d63c in redisplay_device (d=0x8effd48, automatic=1)
    at redisplay.c:7114
#43 0x0821dab9 in redisplay_without_hooks () at redisplay.c:7160
#44 0x0821dda0 in redisplay_no_pre_idle_hook () at redisplay.c:7231
#45 0x0821dd54 in redisplay () at redisplay.c:7213
#46 0x08104303 in Fnext_event (event=150382316, prompt=137821456)
    at event-stream.c:2266
#47 0x080b9fef in Fcommand_loop_1 () at cmdloop.c:622
#48 0x080b9dc8 in command_loop_1 (unused_dummy=137821456) at cmdloop.c:528
#49 0x080eac74 in condition_case_1 (handlers=137818888, 
    bfun=0x80b9d97 <command_loop_1>, barg=137821456, 
    hfun=0x80b99a9 <cmd_error>, harg=137821456) at eval.c:2143
#50 0x080b9ac1 in command_loop_3 () at cmdloop.c:285
#51 0x080b9ae3 in command_loop_2 (unused_dummy=137821456) at cmdloop.c:298
#52 0x080ea5e5 in internal_catch (tag=137558080, 
    func=0x80b9ad8 <command_loop_2>, arg=137821456, threw=0x0,
thrown_tag=0x0, 
    backtrace_before_throw=0x0) at eval.c:1723
#53 0x080b9bf8 in initial_command_loop (load_me=137821456) at cmdloop.c:336
#54 0x080e3ca7 in xemacs_21_5_b31_i686_pc_linux (argc=2, argv=0xbfb4f544, 
    unused_envp=0x0, restart=0) at emacs.c:2738
#55 0x080e44e5 in main (argc=2, argv=0xbfb4f544, unused_envp=0xbfb4f550)
    at emacs.c:3186

Installation file attached.