Continuity mistake: When Doc. Ock exits the hospital, the cab in front passes by the same zebra crossing twice.
Continuity mistake: After Spider-Man stops the train and faints, four hands comfort him. A frame later they've changed positions.
Continuity mistake: Before Spider-Man saves the kids from being run over, they're running and are inches away from the road. Five seconds later, after several flying scenes, they've moved 5 metres behind and are running towards the road again.
Continuity mistake: When Peter crosses the campus, its avenue is almost empty, but in the close-up it's filled with people.
Continuity mistake: When Peter stops his bike by the pizza man, a guy entering the store on the left disappears.
Continuity mistake: When Peter bounces into Professor Connors, the passers-by behind are not continuous between shots.
Continuity mistake: The wrinkles on the sticker on Peter's helmet change between shots.
Continuity mistake: Before Spider-Man saves the children, the black truck runs by the same yellow cab twice.
Continuity mistake: After Spider-Man scolds the kids for playing on the street, the boy's hair swaps from messy to brushed. When Spider-Man leaves, it changes back to messy.
Continuity mistake: When Peter stops his motorbike by a grocery store, a group of children suddenly appear out of nowhere.
Continuity mistake: When Octopus's house is about to fall on Mary Jane, the box to her left swaps to being on the right side in the front angles.
Continuity mistake: Mary Jane runs in her wedding dress by a fountain in an empty park. A frame later it's packed with people.
Continuity mistake: When Peter and MJ are in the cafe, before Doc Ock attacks, the same sequence of cars and pedestrians outside the window goes by several times. Pay particular attention when the taxi goes by.
Revealing mistake: After Tobey Maguire stops the train, he is stopped from falling by the passengers on the train. As he pulled back into the train, the vertical metal hand rail on the left-hand side bends and shakes like jelly when it is gently touched by the male passenger as he pushes Tobey into the train even though the hand rail is still intact and fixed to the train. This is probably used so Tobey would not be injured while pressing up against the train during the filming of the scene.
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.