Seems like there's a bug that prevents a level from completing if you don't collect a single bubble... at least there is on level 18 (I think that's the one, it's the level where you start as steam under a platform moving diagonally downwards). I haven't noticed this on other levels, but then again other levels aren't as easy to complete without getting the bubbles. The animations keep on playing, but the character is gone, and there's no way to restart or access the pause screen. For this level specifically that's a real pain, but overall it was a fun game. Lots of levels, variation; a good challenge!