Indeed someone might even find some sort of exploit (not in the bitcoin protocol, but in our economy as a whole) in the future by figuring out how to constrict the money supply at critical moment.
So someone would spend their money, and while that is confirming, they will are not able to re-spend their money again, thus locking their coins. Yeah, great exploit.
It's like throwing your gold into the Ocean and saying "haha, it's now locked, you can't use it, economy. Take that!"