Alright, I thought I got this problem to go away, but it's back and worse than before. In debug mode, the problem is these items that come out of blocks you can hit. Several events are used to control their movement when you hit the blocks, but since the items are created during run-time, the debugger complains that no such object exists right BEFORE you hit the block, which is right BEFORE the item is created. When that message pops up, I click "OK", and suddenly the screen is completely white. I'm still playing, there's still sound effects, I just can't see anything at all. If I run around and get myself killed, it goes back to normal upon restarting, and the message doesn't come up again.
Now, the problem is ALSO occurring upon restarting the layout when I die. It doesn't happen every time (normally I have to die twice before it comes up), but it's still far too often to be acceptable. I'm not using any transitions, so I don't know what else it could be. I'll try disabling groups one at a time, but I don't think that'll change much.
EDIT: The problem ALSO happens in debug mode if you're using the scroll bar and hold onto it for more than a few seconds.
This has to be a bug...