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.
Continuity mistake: When Zack is in middle school, his teacher, Miss Bliss wants to talk to his father. At that time his father's name is Peter Morris and he is an older man and unmarried. When Zack is in high school and Mr. Belding wants to talk to him about the ski trip, his name is Derrik Morris and he has a mother.
Continuity mistake: In the scene where Max has Zack bring Kelly a Coke, you can see the glass only has one straw in it. You can verify that it's simply not two straws stuck together when he lays it on Kelly's table. As he lays it down, the straw swoops around the outer perimeter of the glass and it can be verified that it's only one straw. However, when Kelly decides to take a drink of it there are now two straws in the glass.
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?
Character mistake: Lisa is upset about being paired up with Screech. Mr. Belding explains to Lisa that the matches were paired up alphabetically. But this is proven incorrect - although Kapowsky (Kelly) and Morris (Zack) are very close, and Slater (AC) and Spano (Jessie) are even closer alphabetically, Powers (Screech) and Turtle (Lisa) should not have been paired. Assuming no other kids names' interfered with these six (in which case the groups would be different anyway). The correct pairs would be Kelly and Zack, Screech and Jessie, then AC and Lisa according to true alphabetical order last names.
Continuity mistake: When the teacher is going through the study fast, you see Kelly stretch her wrist as it hurts from writing so fast. While she is stretching, behind her you see Zack putting on his headphones. A few seconds later the camera is on him and he is putting the headphones on again.