(at around 25 mins) After Peter clobbers Flash, a fat boy with glasses can be seen standing in shock at Peter's abilities, then clapping and smiling, then standing in shock again between shots.
(at around 1h 7 mins) During the World Unity Festival, when the Green Goblin throws his pumpkin bomb at the balcony full of board members, as the windows blow up, Harry can be seen running away from Mary Jane and toward the area he will later have his head hit. In the next shot, Harry is seen still standing next to Mary Jane.
(at around 29 mins) While Mary Jane is talking to Peter Parker behind her parents' house, her trousers repeatedly move up, then down, then up again, hiding, then revealing her underwear.
(at around 22 mins) In the cafeteria Peter web-slings a tray, with hardly any corn on the plate. Then Peter flips it onto Flash and suddenly there's a whole lot of corn on his back.
(at around 1h 50 mins) Mary Jane's hand disappears briefly from Peter's face at the cemetery.
(at around 27 mins) When Peter is trying to figure out how to shoot his web across to the crane, there is a billboard that says tune into 98.8 FM. In the United States, FM radio stations do not end in even numbers.
(at around 23 mins) When Peter is first becoming aware of his "spider sense" in school as Flash is approaching from behind, he sees a fly and a paper airplane moving in very slow motion. The fly has two wings when in reality, flies have two sets of wings, the main wings plus a smaller "halter" wings underneath (to maintain balance in flight), for a total of four wings.
(at around 15 mins) Before going into the vapor chamber, Norman announces to Stromm that he's drinking a vial of prochlorperazine, which will serve as a catalyst for the human performance enhancer. Prochlorperazine is better known as Compazine, a medication formerly used to control extreme nausea and vomiting--it wouldn't have helped the enhancer at all.
The army general's National Defense Service Medal ribbon is missing a star. He should have a star on it for a total of two awardings, one for the Vietnam War and one for the Persian Gulf War.
(at around 1h 23 mins) When Spider-Man saves the baby from the burning building, it's amazing that the baby and the blanket are so clean - no soot, smoke damage, charring, etc. The firefighters have obvious soot around their noses from trying to breathe in that area. Did Spidey manage to give the baby a bath before presenting it to momma?
In several scenes newspapers are shown which contain a few words repeated over and over, instead of actual text. This is probably deliberate, as Spider-Man comic books often have images like this.
(at around 20 mins) When Peter is walking behind Mary Jane, on the way to the bus, he stops and talks to himself. over his left shoulder is a car parked, with a Massachusetts license plate. Cars from many different states visit or pass through New York on a daily basis.
(at around 1h 10 mins) When Spider-man swings MJ to safety and we see them swinging close-up, not only is MJ's hair blowing in the wrong direction, but Spider-man's head, neck and shoulders appear to be stiff as if it were a mannequin she is holding on to for the sake of the shot.
(at around 26 mins) When Peter Parker first discovers he can climb up walls (while in citizen clothing), he tests it out by climbing up a building in an alley. During this scene, you can clearly see his clothes hanging out forwards, indicating that the scene was filmed with him crawling along a floor horizontally.
(at around 40 mins) At the wrestling arena, you can see men putting giant chains and padlocks on to lock the cage, but when Spider-man is declared winner, the cage goes up and opens when no one has removed the padlocks.
(at around 46 mins) When Spider-Man is chasing the carjacker, a brief shot from the inside of a police car shows they are moving at only 15mph. This is not nearly as fast as the obviously sped-up sequence leads us to believe.
(at around 1h 45 mins) When the Green Goblin is trying to stab Spider-Man with his Trident, one of its points bends.
(at around 1h 7 mins) When the Green Goblin says "Hello, my dear" to Mary Jane, he's mouthing something else.
In several scenes, the Green Goblin's mouth (which is visible through the mesh in the "mouth" of his mask) does not move while he is talking.
(at around 5 mins) At Columbia University, one of the two girls say "Hey MJ!", yet neither one's lips move.
Several times, when Spider-Man speaks or even screams, his masked mouth/jaw does not move.
(at around 1h 8 mins) At the Times Square battle, when Green Goblin grabs Spider-Man's hand and says, 'impressive' his mouth doesn't move but stays shut.
(at around 1h 7 mins) When Mary Jane is trapped on the balcony you can clearly see the metal mechanical supports that hold the wall.
(at around 10 mins) When Peter gets bitten by the spider, the camera pans over to a TV screen showing the spider DNA. You can see a camera man's head peek from behind the camera in the reflection.
(at around 51 mins) The tropical orange flowers behind Peter and Norman at the graduation scene are native to California and couldn't possibly survive in New York (although they could be fake).
Newspaper photos almost always have the credited photographer listed next to them. Therefore, Green Goblin/Norman Osborne would have no reason to go the Bugle and integrate J Jonah Jamison to find out who took Spider Man's photo. He should know it was Peter Parker.
(at around 18 mins) After Parker wakes up the morning after getting bitten, a boom mic is seen at the top of the mirror as he is looking at himself.
(at around 42 mins) Peter won the Bonesaw wrestling match by knocking him out, but when the fight promoter stiffs him, the promoter claims he pinned him for the victory.
(at around 22 mins) MJ tells Peter in the cafeteria that she noticed his eyes are blue. She says, "I didn't notice without your glasses." Clearly she meant she didn't notice his blue eyes when he had glasses.
Green Goblin asks Jameson "Who's the photographer who takes pictures of Spider-Man?" In close-ups of the Bugle, Peter Parker's name is clearly listed as the photographer.
(at around 6 mins) During the science trip where Peter is bitten, Harry says that the microscope is the "largest electron microscope on the eastern seaboard". However, the microscope images shown are taken from a phase contrast microscope not an electron microscope. (Phase contrast gives less magnification but lets you work with live material, for example, the cell being manipulated in the image shown on the film).
(at around 32 mins) While Peter is looking at ads for used cars, he sees an ad for a "Triumph TR-3". Above the price it says, "Drives it home!" instead of "Drive it Home!"