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)
And Then There Was Shawn - S5-E17
Continuity mistake: After Eric rips the receiver and cable off the pay phone, at first he is holding the receiver in his right hand and the cable in the left. A couple of shots later, the receiver and cable have swapped hands. (00:15:15)
And Then There Was Shawn - S5-E17
Continuity mistake: When Shawn is grilling Kenny about asking to borrow Topanga's pencil, sometimes there is a female student sitting directly behind Angela with an open book on her desk, and other times the desk is unoccupied altogether. (00:00:21 - 00:02:40)
And Then There Was Shawn - S5-E17
Continuity mistake: When Feeny appears in the hall, he has his hands in front of him, and he is holding them. When Shawn touches him on the shoulder and Feeny falls on his knees, his hands are still in the same position, but when he is shown lying on the ground, his arms are now be his sides. Feeny was dead, so there is no way that he could've moved his arms.
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.