Weird Unreproducible Error in 21.1

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) map-extents(#<compiled-function (e ignored) "...(2)" [e] 1> "-----XEmacs: *scratch* (Lisp Interaction)----L5--C0--All------------------------------------------------" 179 179 nil nil balloon-help) balloon-help-motion-hook(#<motion-event 1034, 785>) run-hook-with-args(balloon-help-motion-hook #<motion-event 1034, 785>) mouse-motion-hook(#<motion-event 1034, 785>) I got the error multiple times, but I can now no longer reproduce this. Does anyone have any ideas? - vin