The Werewolf Fakeout
I think I got the idea from an old Endless Quest book. If it works right it goes like this. The PCs are attacked by several wolves (usually three). One runs away when it gets badly injured. The others fight to the death. The ones that die transform into people and the PCs realize they were werewolves.
A little way down the road the PCs find a badly injured traveller. If they help him, he says he was also attacked (by something other than wolves, so that the PCs don't suspect him of being bitten and infected). He ends up travelling with them somehow (they offer to escort him to a healer, he asks to travel with them to the next village, etc). He is actually the werewolf that got away, and he turns on them when he believes he has the advantage.
It fools the players about half the time.