Continuity mistake: When Peter goes to Harry's house to fight him, between shots, Peter's hair moves from over his eyes to the side.
Continuity mistake: When the cop is walking on top of the truck, we can briefly see that the tarp is covering part of the back right side of the truck. In the next angle it's only covering the front right side.
Continuity mistake: When Mary Jane reads the critics reviews, the back shot shows her holding the paper from the bottom. The front shots shows her fingers on the middle top part.
Continuity mistake: In the scene just after Harry regains his memory and his father tells him "first we attack his heart", we see MJ at the door of her apartment, in the first shot you can see that her handbag is on her left shoulder but in the following shot from a distance her handbag is now on her left forearm.
Continuity mistake: Near the beginning of the film just after Peter has knocked Harry off of his jetboard, Harry falls onto the alley floor where you can see large amounts of puddles where Harry lands, yet in the following shots where Peter is trying to revive Harry, some of the puddles have vanished.
Continuity mistake: When Venom blows the cab's roof, there's a back shot of the cab from above and MJ is nowhere to be seen. The shot is clear and long enough to show the back seats. Furthermore, MJ is wearing a white shirt and, since it's night time, there's enough contrast to show that she is not crouching, nor hiding, but is simply missing from the CGI shot.
Continuity mistake: From a back shot, when Peter Parker is watching the screen with the promo about his life, the kids are standing still next to him. From the immediate front shot, the kids are arriving and taking positions.
Continuity mistake: During Venom and Spidey's final battle, the amount of tubes on the floor diminishes drastically between shots.
Continuity mistake: When Spiderman lands Gwen and leaves, from the back shots her hair stirs violently because of the wind. From the front, it is nicely combed and the wind is not as strong.
Continuity mistake: In the shot just after Green Goblin is done dispersing both the super villains and offers his hand to help Spider-Man up, watch Green Goblin's face closely when he is first shown from a distance, when his face mask has just rolled up. The right side of his face, which is supposedly disfigured (due to his earlier fight with Peter in his flat), looks normal and smooth. Then in the following shot (after the camera goes back on to Green Goblin from Spider-Man lying down looking at him) when the camera shows Green Goblin from close (while Spider-Man slowly gets up at the forefront), his face is shown disfigured again.
Continuity mistake: The Hawaiian doll on the hanging cab moves around between the first and second shot.
Continuity mistake: When the two models stare at the steel beam, there's an employee sitting next to the model with the green shirt. In the wide angle he dissapears. If he had run away he would've been seen because he was standing very close to her.
Continuity mistake: Close to the middle of the film when Harry returns to his mansion with Peter, the butler hugs him on his left shoulder yet when the shot changes he's hugged him on his right, this is so you can see Harry's (James Franco) face not the butler's.
Continuity mistake: When Peter's looking at himself in the mirror in his apartment after yelling at the landlord, his hair's down in the first shot we see of him, but swept to one side when we see the reflection.
Revealing mistake: When Venom knocks Spidey off the cab, he then jumps on Spidey's chest. During that moment, Eddie's face is an obvious, cheap CGI (slow motion can help but isn't essential).
Continuity mistake: At the jazz bar, the amount of people by the bar and their positions (close together/apart, seated/standing up) is inconsistent in every single shot.
Continuity mistake: During the whole Harry-in-the hopsital scene the tube on his chest keeps changing positions all the time.
Continuity mistake: About to kill Venom, when Spidey is straining to break the web around his wrists, a fringe of hair falls on his face. A shot later his hair is combed and looks hair-dried.
Continuity mistake: In the scene where Brock is in Jameson's office showing him the picture of Spider-Man, Peter walks in and hands Jameson a picture, in this shot we can see Brock looking at the picture of what Peter took, and in the following shot Brock is looking directly at Peter asking him "how did he get that high".
Continuity mistake: When Peter is reanimating Osborn, Harry's head keeps swapping form looking upwards, sideways, slightly tilted. between shots. Peter's movements are not that brusque to produce the changes.
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