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: After the meeting with Zack's "fake Dad", Mr. Belding informs Zack he wrote a letter to see the Dad again... However when the "real" Dad sees the fake "Belding" (whom the Dad didn't know was fake), the Dad never questioned about the "seeing him AGAIN" letter when this real Dad never met Belding before. In addition, wouldn't the Dad question this letter to Zack anyway?
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.
Plot hole: Zack was told by Slater that Kelly was excused from going to detention by Belding because Belding found out she saved Screech's life... Yet only 3 students knew about the fake "choking" stunt - Zack and Screech did this stunt to get Kelly into detention (so they wouldn't have said anything), and Kelly unlikely would say anything as generally students will say anything to get excused and usually are ignored by the teacher or principal who use authority to make the student's detention stand. So who else could have told Belding?