1) This one everyone has probably already heard of, but Mr. Hall, (Cher's debate teacher), calls her "Cher Horowitcz", but later in the movie when they show her father's name on her report card, it says, "Mel HAMILTON". It also says that on the credits. I doubt Cher would have a different last name than her biological father.
2) In the scene where Cher and Di, (Alicia Silverstone and Stacey Dash), go into the office to see Ms. Geist, their social studies teacher, Travis, (Breckin Meyer) is there, talking to Ms. Geist. Ms. Geist says to Travis, "Next class let's try to remember to bring our textbooks". However, later in the movie, Travis asks Cher, "You have Smith, right?" (I don't know if that was the teacher's name, I can't remember it), but then Cher says, "Geist". From the earlier conversation, didn't it sound like Travis has Ms. Geist, too?
3) This is another one everyone has probably heard of, but here goes: In the scene where Cher, Dionne, and Tai are in the restaurant and that hot waiter walks by, if you look closely at Cher, you can see her either mouth the line, "Ugh, I hate muscles", or she already said it and was dubbed. Anyway, she says that with a disgusted look on her face, but then a milisecond later she says happily, "Survey says?" And THEN she says the line, "Ugh, I hate muscles". I know it doesn't sound that bad here, but when you see it on the movie, it obviously looks like a mistake.