And Then There Was Shawn - S5-E17
Revealing mistake: When Shawn unmasks the killer and it is revealed to be himself, you can see that the body double that does the unmasking has sideburns and extra scruff on the back of his neck, making him noticeably distinct from Shawn. (00:18:17)
Revealing mistake: When Topanga is picking up the letter off the ground, you can tell the snow isn't real because it doesn't behave like real snow and run together when she scrapes the letter against it. It looks more like shredded paper being swept aside. (00:20:38)
Answer: Topanga's middle name is never specified. Considering her sister's name is Nebula Stop-the-war Lawrence, Topanga's is probably something equally unusual.