Buffy: The Vampire Slayer - Just Because I'm Unconscious Doesn't Mean I Can't Fix My Shirt
This slip up takes places toward the middle of the episode when Buffy is starting to lose her grip on reality. She attacks Xander, throwing him down the stairs where he promptly loses consciousness. However, when he first lands on the ground his shirt has been pulled up far past his waist so that his back is exposed. They then pan away for a split second to show Willow on the floor. When they pan back to Xander his shirt is magically pulled all the way back down, completely covering his skin, even though he supposedly hasn't moved since he's unconscious. It's a pretty hard to miss mistake.
The episode "Normal Again" from season six and your eyes