Honestly, one of the best games I've ever played. However, I found this glitch to get out of bounds in 4-3...
I walked around for a bit and found that you could fall off the edge and endlessly fall and that you could also see some of the underground areas of the level beneath. Also, you can walk around into furniture (such as the TV), but since they have no collision you can just walk into them and see the inside of them.
Other than this, absolutely great game and I can't wait to see the next Polly the Frog game (if it releases).