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.
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).