Revealing mistake: When Topanga emerges from the kitchen with more ice for Cory, instead of seeing the kitchen through the open doorway, you can see a wall with framed pictures, suggesting another set was there in place of the kitchen.
The Heart is a Lonely Hunter - S3-E15
Revealing mistake: When the news director is flipping through the booklet that is Arthur's résumé, you can see the pages are blank.
A Kiss is More Than a Kiss - S3-E14
Revealing mistake: When Cory puts the talking teddy bear that Topanga gave him in the trash compactor, the bear screams and Alan and Amy come down to investigate. Amy opens the trash compactor door and pulls out a bear crushed into a cube, but if you look closely, you can see the original bear still in the trash compactor unharmed.
Revealing mistake: During the montage of Eric and Frankie's game of pool, at one point Frankie hits two balls that spin towards a pocket in unison. When both balls come to a stop, you can see a dent in the green ball where a wire or other device would have been inserted to keep the balls linked together.
Revealing mistake: When Amy steps into the living room and sees Reginald, you can see a plain white wall behind the door that leads into the kitchen, when you should be able to see the kitchen itself, which would suggest another set was placed there and they didn't move it for the filming of this scene.
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.