Continuity mistake: Kelly said her youngest brother can burp his whole name Kenneth Leonard Kapowski in one burp. However in the previous year, Kelly had an infant brother named Billy. According to this info, unlikely Kenneth would be younger than Billy.
Plot hole: Lisa, Zack & Screech all talk to JB as if they have known her since she was little. However, they have only known Slater for approximately four years, so them knowing her from a little girl, as implied, wouldn't be possible.
Plot hole: When the gang is helping Lisa earn tips at her waitress job, Slater says the more people she waits on the more tips she will make. So they take the food away from the customers before they even get to eat it. How in the world would she make 37 dollars in tips with service like that? Horrible service like that would earn her no tips at all.
Factual error: It seemed like they had several senior years in the show? They had The Prom year when Kelly, Jesse, were there. Then they had senior year when Tori was on the show when they did the senior Class Rings.