Whatever Zabini cast wore him out a bit. It was something taxing, and I rather got the impression from the way he looked that it wasn't going to be an amusing sort of curse.

Neither was Crabbe's, but there are much more dire Dark objects out there than ones that petrify your wand hand, so. Anyway. That key Zach Smith got, no one in the class made that, and I didn't see anyone with Zabini's.