Grandma Was a Rolling Stone - S1-E7
Revealing mistake: When Eric first meets Mr. Feeny's niece, he throws a bag of mulch. When the mulch lands, you can see the fake grass move, and the fake background swings a little bit.
Cory's Alternative Friends - S1-E4
Revealing mistake: While Cory and Topanga are handcuffed together, trying to convince their classmates to sign a petition to keep Mrs. Rosemeade, Cory is seen mouthing the lines the others are saying while waiting to say his own.
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Revealing mistake: When Cory writes "Hey Dude" on the chalkboard, you can see how it had been written and erased several times, indicating previous takes.
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.