The Heart is a Lonely Hunter - S3-E15
Other mistake: Dana exits Chubbie's and sees Shawn has set up a picnic for her. She remarks that it's the middle of winter and Shawn is shaking from the cold, but her breath is not visible.
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.
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.