Vin Shelton writes:
 This afternoon I got the following error under a freshly-started
 21.1.6.  I created a second frame (C-x 5 2) and stretched it out so it 
 was nearly full-screen width.  Immediately after resizing the window
 (from the fvwm2 window manager), the mouse tracked across the
 rightmost edge of the modeline and I got the following error:
 
 Signaling: (args-out-of-range "-----XEmacs: *scratch*      (Lisp
Interaction)----L5--C0--All------------------------------------------------" 179 179)
Thanks for this report.  The bug is in balloon-help.
generated-modeline-string is a snapshot of the modeline at last
redisplay.  balloon-help uses the value of this variable to
determine whether a help balloon needs to be popped up over part
of the modeline.  When you resized the frame,
generated-modeline-string became out of date.  So when the mouse
passed over the uncomputed part of the modeline, balloon-help
essentially ran off the end of the string.