It happens when you hit enter TOO fast, I've had it happen a few times, its annoying, but not much you can do. When something is time critical, like the behests, especially when surrounded by 30 people trying to get in, crap happens sometimes, and you just have to live with it.

Think of it like the spawn bug, where you can hit an enemy first, but the other guy tags it..
hate it when that happens on NM's....