Revealing mistake: In the scene where Peter saves the child (Venus Lam) from the burning building, right after the jump over the floor opening, the child stands up to help Peter we can see she wears knee pads under her pajamas.
Revealing mistake: As Mary-Jane is being lowered onto the crane after Spidey has saved her, you can see that her dress is being held together to stop the split up her leg opening up too much. However in the next shot when she runs down the steps towards her fiance, the dress is flapping around and the split is wide open.
Revealing mistake: In the scene where Spider-Man is pushed inside the train by Dr. Octopus and grabs the bar then leaps to the next: The passengers are all looking where they've been told Spider-Man is supposed to be, but none of them are actually looking at him, revealing that he is a CGI image and not an actor.
Revealing mistake: In the scene right after Peter and MJ swing off and they are on the web, Peter is crawling toward MJ. You can see on his right thigh that where there is a tear mark that his skin isn't showing through, but instead there is a piece of flesh toned material. (It bends unlike the skin on the thigh would)This was probably done so the Spidey suit would still remain tight to Peter's body. (01:53:00)
Revealing mistake: When Mary Jane is being pulled toward the tritium when Doc Ock has her, the shot is taken from her feet up to her head. If you look where her dress ends, you can just barely see that instead of having the regular open dress, it is switched with shorts of the same type so you can't see under her dress.
Revealing mistake: During the final battle, when Spidey tells Doc Ock "we have to give up the thing we want the most... even our dreams", watch the tentacle behind Doc Ock's head. Right after Spidey says "even our dreams", the three "claws" on the tentacle close, and you can see the tip of the bottom "claw" pass through the tip of one of the other claws, revealing a blunder in the rendering of the digital effect. (This is visible in the DVD release of the theatrical cut of the film. It was corrected in the "2.1" edition, and may have been corrected in the Blu-Ray release, but it's still plainly visible on the original DVD release).
Revealing mistake: After Tobey Maguire stops the train, he is stopped from falling by the passengers on the train. As he pulled back into the train, the vertical metal hand rail on the left-hand side bends and shakes like jelly when it is gently touched by the male passenger as he pushes Tobey into the train even though the hand rail is still intact and fixed to the train. This is probably used so Tobey would not be injured while pressing up against the train during the filming of the scene.