On the third level I walked through the walls going horizontally, I think I was in mid-air from jumping but I'm not sure if that was a critical part of making the bug happen. The guy fell down through the bottom of the screen and didn't die or anything, just fell off the screen, and the game at that point was pretty much locked for all practical purposes (even though the guns were still firing) and I had to reload the page. Happened twice.
If it's a fixable bug then by all means fix; if it would be very difficult to find and/or correct then maybe just make the guy die if he falls off the bottom of the screen so at least the game doesn't get stuck.