Many lettering "mistakes" are just choices one doesn't agree with, or stuff that could have been better placed. This? Straight up mistake; a chopped off balloon that should have gone against the panel border; or, judging by the rest of the art and the other balloon, not chopped off at all.
What's weird it's that the letterer here is Chris Eliopoulos, one of the letterers I look up to, and one of the best in the business out there right now. Heck, I can even spell his last name right without checking! This mistake is probably related to the fact that Chris letters like a third of the Marvel books every month (with, like another third going to Joe Caramagna, and everyboy else splitting up the last third).
(From Magneto: Not A Hero #1, November 2011).