Continuity mistake: When Rachel tackles Jack onto the couch and is straddling him, Jack's head ends up resting on the green and yellow pillow on the cushion closest to the door. After Jack says "She's sittin' on me, man," the shot changes and his head is now on the middle cushion. (00:16:06)
Other mistake: When Mr. Feeny is telling the story about Topanga and Cory in the first grade, he mentions that Shawn told him to leave them alone. However, the season 3 episode "City Slackers" established that Shawn wasn't in school with Cory and Topanga until he was around 12, and that he was in and out of 5 schools in different states before then. (00:18:40)
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.