After you exceed around 2.14 billion in the score for the endless hard level, the score becomes negative, so new points make the number increase toward zero.  I made it to level 248 or so with a score that should have been close to 5 billion, but I guess the score doesn't count once you reach this glitch.  This is the max score on the leader boards, so it looks like others have encountered it as well.

I hope this bug is fixed, as I think it is a bit lame to play for 5 hours and not have your score recorded at all, let alone an awesome one.

Props to dave for an awesome game.