I happened to be using a "slightly old" version of CVS Emacs
GNU Emacs 22.0.50.3 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
of 2006-01-19 on kfs-l.imdomain.dk
and went away from the screen for an hours so, and when I returned,
Emacs was stuck in a non-responsive mode with a "blank" emacs frame.
I attached to it with GDB, and this is what I got:
bt
#0 0x403d8122 in select () from /lib/libc.so.6
#1 0x402bbdc4 in _XlcPublicMethods () from /usr/X11R6/lib/libX11.so.6
#2 0x40210ee8 in _XFlush () from /usr/X11R6/lib/libX11.so.6
#3 0x402124fd in _XReply () from /usr/X11R6/lib/libX11.so.6
#4 0x401ff722 in XInternAtom () from /usr/X11R6/lib/libX11.so.6
#5 0x4066bf31 in _XimExtension ()
from /usr/X11R6/lib/X11/locale/lib/common/ximcp.so.2
#6 0x4022e515 in XFilterEvent () from /usr/X11R6/lib/libX11.so.6
#7 0x080e59f5 in x_filter_event (dpyinfo=0x8621100, event=0xbfffd8a4)
at xterm.c:5586
#8 0x080e7ad1 in XTread_socket (sd=0, expected=1, hold_quit=0xbfffe974)
at xterm.c:7016
#9 0x0811a03b in read_avail_input (expected=1) at keyboard.c:6712
#10 0x0811a232 in handle_async_input () at keyboard.c:6858
#11 0x0811a275 in input_available_signal (signo=29) at keyboard.c:6900
#12 <signal handler called>
#13 0x403d8122 in select () from /lib/libc.so.6
#14 0x00000010 in ?? ()
#15 0x081164a6 in kbd_buffer_get_event (kbp=0xbffff0a0,
used_mouse_menu=0xbffff39c) at keyboard.c:3935