Continuity mistake: During the scene where Peter is on his way to the play, he stops the cop car flying through the air by catching it in a web, but the glass shards and debris which are flying along with the car are not in the web anywhere, nor do they hit the people in the path of the car.
Factual error: In the scene where Peter is saving the children from the burning building, there is no smoke from the fire, when there should be black smoke billowing out the windows. He wouldn't be able to just stand up and walk through the building. But in order for the scene to play out on camera, the area is smoke free. A fire in an apartment or office building will include combusting of large amounts of plastics (computers and so on). So unless there is nothing but alcohols in the building there will be immense amounts of smoke.
Continuity mistake: As Mary Jane is reunited with John Jameson after Spider-Man lowers her down from the crane, one shot shows tears along Spider-Man's suit along the left leg while he is crouching. However, in a later shot, he stands up to swing away and the tears are now running along his right leg.
Continuity mistake: After Peter came out of the deli, you could see and hear people running around screaming hysterically. But when the shot showed Peter's glasses falling to the ground in slow motion, you could see people were just walking down the street leisurely.
Continuity mistake: As Peter is walking along the top of the building, about to do his "I'm back" jump, he is about three quarters of the way past a skylight on the roof. Then the shot changes, and he is just walking up to it.
Revealing mistake: When Mary Jane is being pulled toward the tritium when Doc Ock has her, the shot is taken from her feet up to her head. If you look where her dress ends, you can just barely see that instead of having the regular open dress, it is switched with shorts of the same type so you can't see under her dress.
Continuity mistake: As Ock climbs the building with Aunt May, a girl is on the phone and feels the trembles. She hangs up the phone and gets up to look out the window. At the desk in front of her, there is an employee helping a customer. As the first girl goes to the window, the customer gets up to leave. The tentacle crashes through the side of the building, the camera pans out and the employee helping the customer is away from her desk, the chair is pushed in and the phone is off the receiver.
Audio problem: Just before Aunt May does her hero speech she tells Peter that he 'Did a very brave thing, telling me, and I'm proud'. There is a pause between each clause of the sentence and the 'Telling me' line is obviously dubbed, the shot is from behind but the sound of the line is different.
Continuity mistake: When Peter is being fired from his pizza place job, there is a clock behind him. While the clock is on Peter the second hand is near the 4. The shot then changes to the boss and then goes back to Peter about 10 seconds later, so the second hand should be on the 6 or the 7, but it is up at the 2.
Continuity mistake: On the way to the play when Spider-Man is chasing the criminals in the car, the passenger is using a shotgun which switches to an automatic weapon which then switches back to a shotgun in a matter of seconds. There is no time for him to switch the guns that fast in the scene.
Continuity mistake: When the car flies past Peter's head, we have a full view of the tire rim which means we are looking at the side of the car. But this view is only possible if the car were thrown in head-first or tail-first, which wasn't the case.
Continuity mistake: At the end of the "I've changed" conversation with Mary Jane, the taxi is right next to her as she turns towards it (you can see its roof next to her face), yet in the very next shot she has to cross the street to get to it.
Continuity mistake: When about to deliver the pizzas, Peter's hair swaps from messy to brushed.
Continuity mistake: When Harry is held upside-down by Doc Ock, his tie is tacked to his shirt partway down (so it doesn't fly in his face). A couple seconds later, on the balcony, Harry's tie is not tacked.
Continuity mistake: While in the bank trying to get a loan, there is a desk with two chairs behind Peter and Aunt May. As Doc Ock throws the vault door in their direction and Peter kicks the two of them out of danger, there are no chairs at the desk behind them.
Other mistake: This mistake can be seen in Swedish theaters: right in the end where Peter Parker says "Thank you, Mary Jane Watson" the Swedish subtitles say "Wilson" instead of "Watson".
Factual error: When Peter calls Mary Jane to apologize for missing her show, he puts in two coins and then dials seven digits. By the time New York City pay phones went from 25 to 50 cents, a caller was required to dial eleven digits for all calls.
Continuity mistake: In the scene where Aunt May is outside her house, packing things into boxes, we see a close-up of her holding what appears to be a large black leather shoe at upper chest level. The next shot is a wide shot of the yard, and now she's holding it in a different position near waist level.
Continuity mistake: The spiderweb that Harry brushes aside is missing from two previous shots: from inside the secret lair looking out at Harry, and from the reverse shot looking into the lair.
Continuity mistake: In the final conversation between Peter and Mary Jane at his doorway, the lighting on his face switches in every shot.
Answer: While it's never mentioned in the film, the most likely explanation is that Harry, as a rich kid, grew tired of "slumming it" and decided to move back into his mansion.