They would get a completely random/hated-by-the-player superpower. Just to mess with them and I'd say afterwards (you didn't say which power you wanted.)
In all seriousness though, I'd probably roll the die between ones that they'd like, those that were useful, those that didn't have to worry about huge setbacks with their powers, some that did, and I'd just go through UP just for good measure.
Sorry, that wasn't in seriousness either. Usually, their power would probably be the one I worked on them with before the game started because to do that while the game is going on... would be rather annoying to do. Especially since my players have never read any of the books unless it pertains to deciding which power they wanted, or reading what they wanted to go with their power.