Kepler

1) Double-click on an actor and change a parameter.
2) Click the add button to trigger the "add parameter" dialog
3) Click Cancel

At this point, you're dumped back to the canvas, and your alteration to the parameter of the actor has been committed. Seems like the preferred behavior is either dumping the user back to the canvas with all changes rolled back, or (even better) just return the user to the normal parameter change dialog after they're done with the add parameter dialog.

I've managed to produce this on the RC3 installer on Windows XP, and on the Kepler trunk on a Mac.