I've noticed that once I play a movie I'm working on I can no longer undo the last change (at least with some changes). For example, if I scale a selected frame, I can undo it as long as I don't play it first. Of course, with some things, I don't really know if I want to keep a change until I've seen it work in the movie. So if there is anyway (without enduring a programming nightmare) to allow an undo of the most recent change even after the movie has been played, I'd love to see that added.