Whenever Spider-Man removes his mask, his costume (both red and black versions) has a turtleneck collar that goes up to the middle of his neck. Whenever his costume is shown underneath his clothes, the collar is much lower.
(at around 1h 50 mins) Before the final battle, when the crowd looks up at Spidey swinging in toward the building, the fireman at the left of the screen picks up the same child three times.
(at around 1h 35 mins) Mary Jane is wearing high heels when she first takes the stage in the Jazz Room. Later on, she is suddenly wearing flats.
(at around 37 mins) When Eddie dashes through the doors to Jameson's office, Miss Brant picks up the phone. In the very next shot she is in the background looking at Eddie and Jameson, but is no longer holding the phone.
(at around 14 mins) When Peter goes to tell Aunt May he is going to propose to Mary Jane, Aunt May starts to hug Peter with her head over his left shoulder. In the next shot from another angle, her head has shifted to Peter's right shoulder.
(at around 33 mins) Captain Stacy is shown wearing a variety of medals above his shield (badge). The green bar with gold stars, and the plain green bar should be worn at the TOP of his shield, above all the other bars, (not halfway down), as they represent the Medal of Honor and the Combat Cross. (The NYPD's Highest and Second-Highest awards).
Several scenes show police officers having an "N.Y.P.D. Police Department" patch instead of the actual patch which would read "Police Department, City of New York". In addition, the real police officers (non actors) can be seen wearing the genuine patch in scenes filmed on location in New York.
The newspaper review of Mary Jane's performance was nonsense. While she does have a weak voice - one with which in real life she never would have gotten the part - it still would have filled the house. For decades musicals have been using concealed wireless mics, which she definitely would have worn.
A character says "Doctor Connor's class" instead of "Doctor Connors's class".
(at around 1h 4 mins) The first time the Symbiote covers Peter's Spider Man suit and "copies" it, Peter isn't wearing the spider man mask, however the symbiote copies the mask exactly, as if Peter were wearing it. However, considering the symbiote affects his mind and amplifies his emotions, it's possible the symbiote copied the suit from Peter's memory.
(at around 1h 35 mins) When Peter takes Gwen to the jazz club and they sit at the table, you can clearly see Willem Dafoe (who died as Norman Osborn in Spider-Man (2002)) sitting and enjoying himself as an extra at the table behind them. This use of major cast members doubling as extras is a habit of director Sam Raimi, which recalls his first low-budget movies.
Eddie Brock is the new guy trying to get a job at the Bugle, but in Spider-Man (2002), Jameson and Robertson talk about a photographer named Eddie who already works for the Bugle. However, they never said Eddie Brock, so "reintroducing" the character does not break continuity.
The main camera Eddie shoots on is a Nikon D50, which is a digital SLR (the LCD screen of the camera complete with "Nikon" logo at the bottom is also visible for a short time in one scene). Although it's a digital camera, it makes a distinct sound, like a mechanical film feed. In fact this is the sound of the reflex mirror moving for each shot, which all SLR cameras (digital or not) make.
(at around 41 mins) When Harry and Peter are playing basketball in Harry's house, Peter hits the vase. When Harry grabs the basketball the ball is apparently sticking to his fingertips. Holding a basketball like this is difficult, but not impossible for a very-skilled basketball player (or someone with Harry's newfound superhuman abilities.)
(at around 45 mins) When the cop holding a shovel is walking on top of the sand in the truck concealing Sandman, you can see the right hand edge of the platform used to stop him sinking into the sand. He stops right at the edge of the platform to raise the shovel.
(at around 17 mins) After Peter leaves Aunt May and is on his little yellow bike, when Harry swoops down to knock Peter off, Peter's red helmet flies off behind him a split second before he is actually hit.
(at around 45 mins) When the police are searching for Sandman in the sand truck, the cover for the truck is tied down by multiple cords, but the officer unties only one cord before the whole thing comes off.
(at around 9 mins) When Peter and Mary Jane are lying in Peter's spider web near the beginning of the movie, as the scene is zooming out you can clearly see a strand of web made out of chain near the bottom of the screen.
(at around 1h 7 mins) When Spider-Man chases Sandman into the storm drain, he tears the grate out of the street, also ripping out concrete and causing cracks in the pavement. When the camera angle cuts to looking up from the storm drain, there is a nice square hole as if the grate were lifted out neatly.
(at around 5 mins) When Mary Jane is singing on stage at the beginning of the film, the audience applauds after the first couple of lines. In the next shot, no one in the audience is clapping, though the applause can still be heard.
(at around 1h 35 mins) When Peter Parker takes Gwen Stacy to the jazz bar, you can clearly hear drums and cymbals when Peter is dancing, but the drummer in the background is only tapping his drumsticks together.
When the jazz band starts to play "Stormy Weather", the first note of the drum part is a striking of the snare drum with a brush, but we don't see the drummer move his arm to play this note.
(at around 1h 15 mins) When Peter starts to place a call on the pay phone before speaking to his landlord, the coins drop directly to the coin return. Then, when he actually hangs up, there is no sound for the coins being returned.
When the scene cuts to Eddie's reaction after Gwen tells him they only had coffee, Gwen's mouth is still moving, but there's no audio from her. Gwen and Eddie have a few more lines in the novelization.
(at around 1h 29 mins) During Peter and Harry's brawl, Peter is grappling with Harry on the floor and when Peter jumps up from lying on his back, an acrobat's cable can be seen on Peter's back.
(at around 1h 3 mins) Shadow of camera briefly visible when the symbiote is getting ready to "attack" Peter Parker on the bed.
(at around 46 mins) When the Sandman throws a car into the street, it clearly hits one car that turns sideways, then goes through the center of the street. Two cars behind the first hit, however, are pushed to the side, and the mechanism pipe that moves the cars is clearly visible.
(at around 11 mins) Flint Marko is said to have just broken out of Rikers Island prison, which is located in New York. However, when Marko finds a stack of letters he wrote to his daughter while he was incarcerated, the return address is listed as Wrightstown, New Jersey.
The film is set in New York but there are obvious shots of Chicago and the Cleveland Terminal.
A part of the movie features the Cleveland Trust Rotunda in the background where one can easily read "Cleveland Trust" inscribed on the Romanesque structure, although the movie is supposed to be set in New York city. Two other unique Cleveland landmarks are also briefly visible - Terminal Tower, and the marquee entrance to Playhouse Square Center.
(at around 1h 45 mins) When Venom/Eddie Brock asks Sandman/Flint Marko to team up, he mentions his history and the reason why he hates Spider-man. How does Venom/Eddie Brock know all this? It's not mentioned.
Peter Parker does not use his Spidey Sense in the first part of the movie (for example when being attacked by Harry).
In Spiderman (2002) Joseph at the daily bugle mentions how 'Eddie' has been working extensively on a job, clearly a reference to Eddie Brock. In Spiderman 3, Joseph tells JJJ that they only hired Eddie Brock (Topher Grace) last week.
Sandman is "dematerialized", along with all his clothes, but not the medallion without any plausible explanation.
(at around 1h 50 mins) When Spider-man shows up for the main fight scene, a lady holding a girl says "look!" and everyone turns around to see him. But the lady and girl are facing the other direction, so she couldn't have seen Spider-man coming.
(at around 6 mins) When Harry is looking down at Peter from the balcony when Mary Jane is singing in Broadway and Peter is singing along, Mary Jane sings one thing and Peter mouths another.
Bruce Campbell plays a French character and states "I am French" but he obviously speaks French with a strong American accent (he is American).