I think the probabilities work out roughly as follows: - 50% Malfoy's Imperius debt. - 20% nothing anyone here has thought of - 10% Something involving the true Patronus(Hermione casting it, etc.) - 10% all the other wacky theories proposed combined - 10% Harry fails and Hermione goes to Azkaban.

I realize that this looks like a list designed to make me not look like too much of an idiot no matter what the result is, but I am not particularly confident, so I'll leave my error bars wide.