Deliberate mistake: When Carrie is hiding in the locker room to record John talking with his friends, it's clear that her camera is behind the vent, and the openings in the vent are closer together then the diameter of her camera lens, however we don't see the vent slats in the camera view or the replaying of the video footage in a later scene. Not to mention the fact that she isn't holding the camera steady. This is impossible.
Continuity mistake: When Carrie hides in the guys locker room she videos John saying "Let's just say, at tonight's away game I'm gonna be scoring more than baskets". In a later scene when Carrie replays the video to Kate he says "I will have you know, at tonight's away game I'm gonna be scoring more than baskets".
Continuity mistake: When Scott is talking to Kate in the school lobby about how he's supposed to be a spy for John, pay attention to the background when they're both staring into each others' eyes. In one shot there are two people by the pillar, 2 seconds later they're gone. Then Kate says "Go, fight, roar" and suddenly those two people are there again. (00:41:25)
Continuity mistake: When Heather is asking John Tucker about whether or not he's cheating on her PE classmates, pay attention to the two handbag straps (one black/one white) on her right shoulder. In some shots the white strap is covering the black one, and in others they're both side by side. (00:13:15)
Deliberate mistake: In Kate's voiceover at the end, she's talking about how it took three days to get the stripper cake out of her hair. But the cake that the bikini-clad girls popped out of was cardboard, which is wheeled away before they bring John's actual cake out (which is what they throw at each other).
Continuity mistake: At the party near the end when John is trying to tell the audience about how it's wrong to lie, pay attention to Beth's handbag. After Kate throws the cake at John, her handbag suddenly disappears. When the food fight gets going, both Carrie's and Heather's bags disappear also. (01:22:40)
Continuity mistake: When the girl is hiding in the locker recording what John Tucker says, she says a few things whilst the camera is recording, but later on when the tape is played at Johns party, we can hear what he said, but the girl who was talking whilst recording, her comments are not present.
Continuity mistake: In the cinema where the genital herpes incident occurs, when the girl that John is with stands up to leave - notice the guy with the afro wearing a yellow t-shirt in the row just behind them. In the very next shot he's suddenly gone. (00:20:50)