Continuity mistake: From the truck, Sandman boosts some cars away. One of them goes zooming straight against a corner on the left side of the street. When the angle changes, not only has the car changed positions to the center of the street, but a man and a car appear perfectly intact and calmed on the same spot where the car was before.
Continuity mistake: When Peter visits Harry at the hospital, Harry says: "The doctor says I had an accident". then he turns his head to the right, facing upwards. In the immediate angle he is still facing left.
Continuity mistake: When Sandman is down in the sewer, the stack of dollar bills on the right has no plastic wrap around. When the angle changes its green colour has faded and you can notice plastic securing the wads.
Continuity mistake: During the crane incident scene when Gwen is holding onto the telephone you can see her sliding down the damaged office floor with her legs almost over the edge, yet in the following shot when the phone's cord snaps, she is a lot further up the office floor and slides down again to the edge.
Continuity mistake: In the scene that Harry yells to Bernard (his butler) to get food because they are having a guest, the first shot of Bernard shows a small table with a vase of white flowers on it. In subsequent shots of the butler within this scene, the flowers are no longer on the table.
Revealing mistake: When Spidey is placing the tubes around Venom, his face is obvious CGI except for some shots of him shouting. Here, a real face (twice as big as it should) has been pasted on the CGI body, looking quite ridiculous.
Continuity mistake: When the steel beam first crashes, Gwen lays very close to the side where it strikes. From the outside, she has moved way to the opposite side.
Continuity mistake: When Gwen is sliding down the office floor, she is facing upwards. Half a second later she is facing downwards.
Continuity mistake: When Peter and MJ are on the web, after he says "I always loved you" she smiles and shows a thick and long strand of hair along her forehead. Half a second later the face is spotless.
Continuity mistake: When Peter exits the theater, a woman with a bouquet of flowers disappears in the wide shot.
Continuity mistake: When Venom kicks Spiderman's mask off, Spidey's fringe swaps from messy to combed between shots.
Continuity mistake: When Peter surprises MJ in the park, just before he receives the key to the city, MJ says "no, I'm proud of you" at this point they hug, MJ hugs Peter on his left shoulder and when the camera angle changes she's hugged him on his right shoulder.
Continuity mistake: The thickness of the bandage on Harry's forehead differs in length through shots. Use his eyebrows as a reference.
Continuity mistake: At the jazz bar, the waitress left to Gwen keeps appearing and disappearing, and changing positions in less that 10 seconds.
Continuity mistake: In the bell tower, the sweat stain on Eddie's t-shirt increases and decreases all the time.
Continuity mistake: The position of the white debris fallen on the taxi's windshield changes between the first and second shot in less than half a second.
Continuity mistake: When we see Peter riding on his scooter after just visiting Aunt May and telling her he's going to ask MJ to marry him, we can see that Peter is wearing his safety helmet with the strap fastened under his chin, yet when the hobgoblin knocks him off of his scooter and into the air the helmet can be seen flying away like it wasn't even fastened up and secure.
Continuity mistake: Before MJ throws the chunk of concrete at Venom, she has her arms raised holding the chunk in the air. In the close-up of Venom's face MJ can be seen in the background, resting calmly in the cab. Then, when back to a close-up of her, she is holding the chunk as before.
Continuity mistake: After Peter smashes his back against a brick wall, watch the left side close to his ear. The first shot shows several bricks missing. Several seconds later the wall is fixed and there's just a tiny chip missing.
Continuity mistake: When Gwen stares at the steel beam, the beam is perpendicular to the window, pointing in a 7 o'clock direction. A frame later, in a wider angle, it is horizontal to the window.
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