Start DrJava, type Alt+A. The undo button becomes enabled.
Choose "Undo previous command" from the Edit menu or click
Undo on toolbar (don't use Ctrl+Z). A CannotUndoException will
appear in the Console tab.
Occurred while using the current CVS copy of DrJava (or earlier
releases, not sure how far back) on Windows or Linux. Seems to
happen for Alt+(any key which is not a menu hotkey).