Continuity mistake: The news reporter is looking upwards and has no camera in front (if she did, a big beam of light would've been on her face). However, when half a second later the angle changes to the news host, the woman is looking down with a camera focusing directly at her.
Continuity mistake: During the very last fight, the bruise on Peter's right cheek keeps increasing and decreasing its redness.
Continuity mistake: During the fight between Harry and Peter at Harry's mansion, just after Peter kicks Harry through the glass window to outside, if you watch closely there is nothing on his right shoulder, then in the following shot a piece of glass appears on his shoulder.
Continuity mistake: Inside the bell tower, when the black slime falls on Eddie he is standing inside the pattern on the floor. In the close-up, 2 frames later, he is standing on the cobblestone outside the pattern.
Continuity mistake: During Venom's 3 second long flight to stab Spidey, moments before he stabs Harry, shots alternate between Spidey and Venom. The stuff behind Spidey keeps changing positions all the time.
Continuity mistake: Inside JK's office, from the back shot the glass of water disappears and the empty mug gets filled with pencils.
Continuity mistake: The blade put against Spidey's throat faces up, then down in less than a second, without Venom moving it.
Continuity mistake: The first time that the crane destroys the office it sweeps desks and furniture, which fall on the people below. However, the second time that the crane strikes, a wide view shows that all of the furniture from the edge remains perfectly placed on the office.
Continuity mistake: JK's cigar when shouting at his secretary disappears from his hand.
Continuity mistake: When Spidey makes Venom explode and watches the metal bars around melted, the one in front is really twisted. Couple seconds later, from another angle, the bar is just sightly bent.
Continuity mistake: The first time we hear MJ's answering machine message, she says something along the lines of "Sing your song after the beep, 'beep'" then we hear the electronic beep. The second time it sounds exactly the same, so she hasn't re-recorded it, except this time her message cuts off before she makes the beeping sound.
Continuity mistake: In the scene where there is a crane accident and they can't shut down the power, the police chief arrives on the scene, watch closely behind him, the police car that pulls up has its back window down, we can see this because a cop has his hand on the doorframe, yet in the following shot the window is up.
Continuity mistake: When Sandman reaches for the silver locket, the shadow keeps changing between shots.
Continuity mistake: Before MJ throws the concrete at Venom, the black web holding the passenger's right side of the cab differs between shots, even disappearing and then reappearing in one of the shots.
Continuity mistake: When Mary Jane brought the newspaper, the main picture was dark and with a bit hidden on the folded side. When she leaves and picks the paper, the picture is very light and has no part folded.
Continuity mistake: When Spidey lands on the hanging cab the red LEDs of the meter are glowing. Seconds later, when the roof is blown away, the meter disappears (either blown out or disconnected), but reappears glowing half a second later.
Continuity mistake: When the bank truck crashes, a yellow cab appears and disappears through shots.
Continuity mistake: When Harry's laying on the alley, his legs swap from bent to straight between shots.
Continuity mistake: When the cop is about to stab the shovel on the sand, the front shot shows a huge jet of sand rising. In the following back shot, for half a second we can see that the cop is standing there and the sand has not risen yet.
Continuity mistake: When Venom hits Spidey away from the cab, Spidey holds onto the edge of the trunk. In the close-up his hand is almost on the center part.
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