XEmacs 21, latest version from unstable, with gnuplot-mode; turn XEmacs toolbars off from the options menu, turn balloon-help mode on (M-x customize-apropos, balloon), but don't turn off the gnuplot toolbar; leave it at it's default configuration. Open a file and set it to gnuplot-mode, then write in a command for gnuplot and highlight it. Fly the mouse pointer over the "region" toolbar button, and the X server crashes, every time, right when the balloon-help window posts. I am using the latest sawfish from unstable also.
I do not know how to find the crash. If you teach me how to do that I will be able to provide more information in the future when things like this occur. How can I observe which program / library the crash occurs in? Any HOWTO out there I should know about or someone should write?