Continuity mistake: When the car crashes in the cafe, it crashes against a column on the left. In subsequent shots the column has no damage at all.
Continuity mistake: Mary Jane holds the wedding card with her left hand. A frame later she is holding it with her right hand.
Continuity mistake: When Peter asks Mary if she wants some chop-suey, the same cab passes behind her twice.
Continuity mistake: In the bank, a desk filled with papers zooms towards Dr. Ock. The papers fly away, but reappear a frame later when he tears the table apart.
Continuity mistake: Spidey and Octopus fight at the tower and a huge chunk of concrete falls on the street below. Seconds later, however, cars are seen moving around, no honks, no crashes, no problems at all, as if the falling rubble had simply vanished.
Continuity mistake: When Spider-Man stops the train his head and chest fall forward due to the momentum, but a frame later - from a different angle - he is standing still, his body against the train structure.
Continuity mistake: Right before Spidey stops the train there's a lot of people inside next to him. Then a wide shot later they've vanished only to reappear a shot later.
Continuity mistake: Spidey throws a huge iron minute hand at Ock which destroys an enormous chunk of concrete and makes loads of debris fall downward. However, a shot later there's a view of the street and the train track and not only is the debris gone from the road but the traffic and people act as if nothing had happened.
Continuity mistake: After the car slams the cafe, Peter watches the street with people running around. The reflection on his lenses has nothing to do with what the opposite angle shows.
Continuity mistake: When Peter/Spider-Man has his first web failure and falls on the pipes on the building's roof, he keeps his hands free in one shot and then his right hand is put on his chest in the next.
Continuity mistake: Kids buying fruit suddenly appear behind Peter when he arrives at the pizza place for the first time.
Continuity mistake: Peter back flips off his moped to avoid the car racing at him. There were cop cars and other vehicles behind that car when Peter jumps, but when he lands only one cop car goes past in the lane next to him and the lane he's standing in is then clear of traffic as he talks to some kids.
Continuity mistake: When Peter first sees the burning building, there is a far away shot and you see the guy with dreadlocks run to a woman. Then it cuts back to Peter as he tries to become Spiderman then remembers he can't. Next shot has the same shot, only this time it's a close up of the man with dreadlocks running to the woman.
Continuity mistake: After the bank scene where Spidey saves aunt May from Doc Ock, while they are on the street the sun changes between shots.
Continuity mistake: During the demonstration of the fusion reaction when Doc Ock is strapping the metal arms to his body, the pins that go in to his spine are shown going in twice from two different angles.
Continuity mistake: In the scene where Peter and Aunt May visit Uncle Ben's grave, Peter's hair is blowing in the wind quite violently in the wide shot but is relatively still in the close up.
Continuity mistake: People behind the pizza guy who stops Peter's motorbike disappear/reappear between shots.
Continuity mistake: During the cafe scene, the spoon under the saucer disappears in the close-up.
Continuity mistake: When Jameson is brought the suit, the position of his secretary's right arm is inconsistent between the front and back angles.
Continuity mistake: The chip on Dr. Ock's nape is 10 centimeters away from his hair. In the close-up it's right next to the hair.
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.