I've had this happen several times, so I guess it's pretty repeatable. If you are editing some text in a multiline text field in form in Safari, hitting undo (command+z) crashes the browser.
Especially frustrating when editing documents in say, a wiki... I was just beefing up my page for the NMC 2004 summer conference, and accidentally deleted a link on the wiki form. I hit undo, hoping to just drop the link back in there, but was greeted instead by a MacOSX Application Crash dialog box.
Doh. Oh, well... Off to rebuild the text from memory. It might actually make some (more) sense this time... ;-)