Deliberate mistake: After the broken suspender clasp smacks Gerry in the face, in the close-up it lands under the dresser, within the wide strip of light reflection. In all other shots the entire area beneath the dresser is in shadow. (00:11:00)
Deliberate mistake: When Evan/August first meets Arthur at the park, after playing, you see Arthur leaving his spot without taking the cable and (presumably) the amp as well.
Deliberate mistake: During the final montage, we see Erik and Joyce in bed together, and the next sequence is of Teddy talking to his baby brother. This was to show that they got back together and had another kid. The mistake is that Teddy would've been a year older by that time, and so would've looked a little bigger, although there is also the fact that the actor who played Teddy (Dakota Goyo) was between 7 and 8 for the filming. (01:47:30)
Deliberate mistake: After Sharpay falls into the pool, when Gabriella dives in it is an obvious stunt double for the actress. (00:16:40)
Deliberate mistake: After Bert is shot in his car, Bobby pulls up beside him and just after Bobby shouts, "Pop, pull over!" the first time, the next tight close-up of Bobby's face is a flipped shot. Note his earring and the scar on his upper lip have switched sides. (01:17:00)