Revealing mistake: While the girl who just broke up with her boyfriend is on the phone with the ex, she is throwing his stuff out the window. As she throws the last couple of CDs out the window, you can see the backdrop of the beach sway a little bit.
Visible crew/equipment: When Eric jumps out of his chair and exclaims "No" during his college acceptance interview, a boom mic can be seen being pulled out of view. (00:19:00)
Revealing mistake: You can tell the ocean visible from the dorm room window is just a backdrop, as there are no waves approaching the shoreline.
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.