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)
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.