Revealing mistake: When Topanga is "naked" under the cover, at one point, you can see a large corner of her bra. (00:02:58)
Revealing mistake: When Cory opens up the curtains of his bungalow to show Topanga the beach, you can tell the background is just a backdrop, as there are no waves approaching the shore. (00:10:36)
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.