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)
Continuity mistake: When Peter and Gwen finish dancing, a waitress passes by them. Less than half a second later, from a new angle, she's repeating the movement. And a frame after that, from a third new angle, she disappears from screen as if she hadn't started walking by.
Continuity mistake: After the armoured truck scene, Spiderman arrives at a roof top, sits down and is about to take his mask off, which is tight around the neck. When the angle changes, before the hand grabs the hood, it already appears loose around the neck.
Continuity mistake: When Venom explodes, from the front shots Peter covers his eyes and nose. From the back he appears covering his mouth and nose instead.
Continuity mistake: During the hanging cab scene, when Venom ties Spidey to a web. A shot shows his back naked. However, when Spidey kills the symbiote, Eddie is wearing a t-shirt.
Continuity mistake: When the model is about to fall, there's a view of the street and the huge traffic jam and chaos from before has disappeared. The cars are driving normally and there's no debris on the asphalt.
Continuity mistake: Before Sandman falls down to the sewer, he lets go a bunch of dollar wads. When he is free falling the money disappears.
Continuity mistake: In the middle of the film when Ursula tells Peter that MJ called back, when she first enters Peter's apartment he is sitting at the computer, look closely here, there is no framed picture of MJ standing against the computer, yet in the following shot a framed picture of MJ has appeared standing up against the computer.
Revealing mistake: When Harry and Spidey fly to save MJ from Venom, Harry's face gets replaced by very obvious CGI.
Continuity mistake: When the symbiote is trapped around the metal bars, its wild movement produces air that moves Peter's hair. However, in a couple of shots his hair is still, turning stirred a frame later from a new angle.
Revealing mistake: Despite the fact that the earlier movies establish that Peter's spider powers give him perfect vision, in numerous scenes throughout this movie (nearly every close up) Tobey Maguire's contact lenses are easily visible.
Continuity mistake: In the scene where Brock and Peter are in Jameson's office and Brock is talking about how he wants a staff job working for Jameson, Jameson waves his secretary out of his office. He starts smoking his cigar in his left hand, yet in the following shot he is now smoking his cigar in his right hand.
Continuity mistake: When the crane strikes for the second time, the wide angle shows the photographer on the floor. Half a second later he is standing tall.
Continuity mistake: When MJ gives Peter the final hug, her left hand is around his nape. Cut to a back angle and the hand is away and starting to move to the nape.
Continuity mistake: When the office floor is collapsing, a wide angle shows all the of the furniture sliding down, except for a desk on the left. Seconds later, while Gwen is hanging by her fingers, dozens of chairs and desks keep appearing from nowhere, including the one that throws her away.
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