Continuity mistake: When Venom is choking Spider-Man and the Sandman is punching him as a giant, Spider-Man's mask is cut and under his eye that is exposed, there is so little mask that it is about to snap. Later in the same scene, there is a lot more mask there.
Continuity mistake: When Peter meets Mary Jane backstage after her first performance, her hands continue to move on and off of Peter's shoulders between shots.
Continuity mistake: During the scene on the bridge where MJ breaks up with Peter, MJ's hair is tied behind her head with a few wisps hanging down and her ears visible whenever she is shown from behind. However, whenever MJ is shown in her own close-up during the same conversation, her hair is down loosely and her ears are completely covered up.
Continuity mistake: During the final fight scene, Spiderman is talking to Mary Jane while Harry is flying around. There is a shot of the Sandman about to clap his hands together around Harry. As the scene cuts to Spiderman taking off to save Harry and cuts back, Sandman's hands are much farther apart than they were before the cut.
Revealing mistake: When Sandman is in the truck of sand and the cop climbs up to catch him, you can see the edge of a platform uncovered. The platform is clearly there so the cop could walk without sinking - he stops right where the platform ends.