Continuity mistake: When Peter lands on the cab, MJ has her fringe all messed up and says "Peter, we'll both die". angle on Spidey, and when back to MJ she's perfectly combed and neat looking.
Continuity mistake: When Sandman leaves his ex wife and daughter, the daughter's position swaps from standing in the middle of a corridor to laying very close to a wall.
Continuity mistake: At the end of the final battle involving all 4 superheroes, Sandman destroys the building's iron structure with a blast of his hand, but in further shots it appears undamaged.
Continuity mistake: When Peter first goes to see Aunt May and tells her he's going to ask her to marry him, Aunt May in the first shot goes to hug Peter on his left shoulder, it cuts away and in the following shot she has hugged Peter on his right shoulder so that the camera can see Peter's face.
Continuity mistake: When the steel beam is sweeping the floor underneath the destroyed office, the model in pink disappears from the wide angle.
Continuity mistake: When the office floor collapses, Gwen and a girl in green are struggling not to fall down. In a close-up, one second later, the girl is replaced by a chair. She can be seen several meters away, next to a guy who magically appeared on scene.
Continuity mistake: During the "Keys to the City" ceremony, when people are chanting for Gwen Stacy to kiss Spider-Man, some of the same extras can be seen standing among different people in the crowd, in successive shots.
Continuity mistake: When the steel beam destroys the office, it breaks a window and a pillar. From the outside, the pillar is intact.
Continuity mistake: In the bell tower, when the slimy thing falls on Eddie, he covers himself with both hands. In the close-up his hands are down and raises just one of them.
Continuity mistake: When Peter is thrown against a brick wall, he produces a big hole around his left shoulder. In the close-up the hole is smaller.
Continuity mistake: Peter opens his shirt wide to see his evil Spidey suit underneath. When the angle changes, a frame later to a close-up of his neck and face, his shirt is closed.
Continuity mistake: Before the cop climbs on the truck, he shields himself on the truck's cabin and you can easily notice the tarp is perfectly tied by a dozen strings. When he climbs up, all the knots magically untie and he manages to pull the tarp away. His buddy couldn't have done it because he always appears with the gun in his hand, covering him.
Continuity mistake: When Jameson kicks out one of his employees from his office for coming up with another stupid idea, Brock sneaks in through the door into his office, but if you look at the black guy in the office he is holding a piece of paper in his right hand and his other hand is in his pocket, yet in the following shot from inside the office, the black guy is now holding the piece of paper with both hands.
Continuity mistake: The Bank truck full stops away from a zebra crossing, but the close shot shows it on top of it.
Continuity mistake: After fighting against the Sandman in the armoured truck, Peter sits on the indent of two brick walls. Despite him not moving, in the second shot he is sitting away from the indent.
Continuity mistake: The first close shot of Harry in bed shows his pyjama opened with a nipple showing. Half a second later, the pyjama appears closed and the nipple covered.
Continuity mistake: During the crane scene, a cop tells the Capt. that the crane is out of control. The captain turns his head completely to talk to the cop, but in the next shot it's partially turned, looking somewhere else, instead of staring at the cop's face.
Continuity mistake: In the scene where MJ and Peter are talking in Peter's apartment about her review of her performance, in the first shot we can see all of MJ's hair is at the back, but in the following shot a huge portion of her hair is now over her right shoulder, this happens between shots and we don't see her flick it over her shoulder.
Continuity mistake: During the crane scene, when the Captain arrives at the destroyed building, a van stops behind his patrol car but, in the immediate shots of the place, it disappears.
Continuity mistake: In the scene where Marco falls into the sand molecular chamber, if you watch closely when Marco looks up in one shot you can see he is wearing a chain around his neck, yet in the following shot when the lights come on the chain has gone, and then the shot changes to the scientists, then back to Marco where the chain has reappeared on his neck. (00:23:05)
Chosen answer: Answered in the comics: Peter's Spider-Sense tells him if someone is looking when he's about to enter/exit the apartment as Spider-Man.
Captain Defenestrator