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.
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.
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.