I don't know if this has been mentioned, yet, but I've run into this bug a couple times.
http://steamcommunity.com/sharedfiles/filedetails/...
I call it the "Great Purple Nothing". There seems to be a level underneath a giant solid colored sprite. I hear enemies moving around up there and when I enter it, I eventually find a hole to fall through.
An interesting thing to note: Quiting the level and then restarting doesn't cure the game of this. I have to restart the entire application.