Continuity mistake: In the train stopping scene, Spider-man's suit begins to rip on the left arm under the strain. For several scenes after (all during the train sequence) this particular rip disappears and reappears at different times.
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.
Continuity mistake: During the scene where Peter is on his way to the play, he stops the cop car flying through the air by catching it in a web, but the glass shards and debris which are flying along with the car are not in the web anywhere, nor do they hit the people in the path of the car.
Continuity mistake: During the demonstration of the fusion reaction when Doc Ock is strapping the metal arms to his body, the pins that go in to his spine are shown going in twice from two different angles.
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.
Other mistake: Sure Spider-Man has super human powers, but his costume doesn't (as we see as it rips and tears quite a lot through the movie) so how come the back of it is still intact and not even dirty after being dragged over the street by a train that's going at high speed?
Other mistake: When Spider-Man and Mary Jane are sweet-talking in the web, her hair blows around freely, whereas considering how sticky a spider web is, her hair should be sticking fast to it. Permanently. After all, the web cannot be sticky enough to hold her body, while not being sticky enough to glue her hair.
Continuity mistake: At the beginning of the film when we see Peter in Jameson's office, the black guy says, "Five minutes to deadline Jonah." Jameson raises his hand to him, but in the following shot it's a lot lower. Also, when he's talking about a food poisoning scare, he raises his hand high up to the right, yet in the following shot, it's far to the left of him.
Continuity mistake: In the elevator scene when Peter/Spider-man first loses his powers, we see a man standing outside the elevator holding a dog on a leash. However, after Spider-man delivers the line about the costume riding up on the crotch, we see the man use both hands to adjust his jacket, and the leash is nowhere to be found, nor is their any indication that the dog is on the elevator.
Continuity mistake: Just after the bank robbery, Doc Ock and Spider-Man are fighting on the side of the building. When some rocks fall down, there is a man (that looks like Stan Lee) pulling a girl away from falling debris. There is a red car to the right of the screen and a yellow taxi to the left, yet in the following shot from a different angle, there are a few more people in front of the red car, and no dust on the ground where the rocks hit the floor.
Continuity mistake: As Peter is walking along the top of the building, about to do his "I'm back" jump, he is about three quarters of the way past a skylight on the roof. Then the shot changes, and he is just walking up to it.