Other mistake: Early in the episode, Amy mentions that she is 7 1/2 months pregnant. This episode takes place around Valentine's Day, which means she would have become pregnant at either the end of June or the beginning of July. However, she revealed she was pregnant around the time Cory, Shawn and Topanga had their senior prom, which would have been around the end of May or beginning of June at the latest, prior to their high school Graduation. So she should be around 8 1/2 months pregnant at the least.
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.