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)
Chosen answer: In the present day, with the United States abiding by more stricter school health laws, for the most part this wouldn't be in school cafeterias as many cannot even have carbonated beverage machines now. However, in the 90s when the show takes place, it wasn't unheard of for there to be coffee machines in the cafeteria of small schools where the teachers eat with the students as we see them do often in the show. The idea being that the cafeteria was small enough so a teacher or hall monitor could catch a student before they could drink the coffee.