Continuity mistake: When Peter visits the doctor, look closely at the skeleton behind him. In the first shot, you can see two spots of light reflected on its skull; in subsequent shots, there is only one spot of light.
Other mistake: When Spider-Man shoots a single web out of each arm to stop the train, in the shots of his reactions, you can see that he is holding many webs in each hand. Then when he shoots multiple webs to try and stop the train, he has the same amount of different webs as before. Obviously they didn't have footage of him just holding a single web, and used shots from later in the sequence.
Continuity mistake: On his last pizza delivery before he gets fired, when it first cuts to the receptionist and the room that she is in, the clock is very high up in this shot, above the receptionist's head, then in the following shot when the receptionist is looking in Peter's direction, the clock is much lower. (00:05:35)
Revealing mistake: As Mary-Jane is being lowered onto the crane after Spidey has saved her, you can see that her dress is being held together to stop the split up her leg opening up too much. However in the next shot when she runs down the steps towards her fiance, the dress is flapping around and the split is wide open.
Continuity mistake: Just after the bank robbery, Spider-Man and Doc Ock are on the side of the building. Doc Ock says "oops, butterfingers." Spider-Man shoots a web to catch Aunt May. In this shot, Spider-Man is on one knee with the other one resting on the wall, yet in the following shot he is standing up and leaning back to pull Aunt May up. (00:48:25)
Continuity mistake: When Doc Ock carried MJ up the side of the building after he kidnapped her, one of the D'Agostino banners was ripped out of the building and fell. But when Peter came out of the deli and looked across the street, the banner was back on the building and the building wall showed no sign of damage from Doc Ock's tentacles.
Continuity mistake: Just after the car goes through the cafe window and almost hits MJ and Peter, they both get up and most of MJ's hair is blown back over her shoulders. However, in the following shots, there is now a lot more at the front of her shoulders, than in previous shots. Throughout the movie, her hair changes position in consecutive shots. (01:27:15)
Revealing mistake: When Doc Ock comes out of the hospital and throws a car onto another one, you can tell that the man in there is just a dummy. He has no reaction whatsoever. He just sits there as if nothing happened.
Continuity mistake: Close to the middle of the film when Peter is in the big room where MJ announces her engagement, when Peter is finished talking with Harry at the bar he gets pulled away by Jameson, Harry then turns around and drinks, yet when it cuts to a shot of Peter and Jameson walking away, look in the background and Harry is still looking at Peter walking away. (00:51:45)
Continuity mistake: Near the end of the film when Peter takes his costume back from inside the Bugle, Peter's boss at the Bugle takes down the note left in a web, in one shot his fingers are half way up the note, in the following shot there right on the edge. (01:29:50)
Continuity mistake: At the beginning of the film, Peter bumps into Dr Connors. Peter has his bag on his left shoulder but in the previous shot you don't see him put it on his left shoulder. (00:08:45)
Continuity mistake: Near the end of the film when we see Peter looking out of his window from his apartment, the girl from across the hall offers chocolate cake. When they are done eating, Peter is holding the fork with his thumb and in the following shot he is not. The fork has moved across the plate a little between shots. (01:17:30)
Continuity mistake: There are two close-ups of Peter's head when he is lying on the floor of the train: one of the shots has a black shoe with white laces very near his head, the other a blue, white, and black shoe, also right next to his head. In the wider overhead shots of Peter, no one standing around him is wearing shoes like that, nor are they standing close enough for their feet to be in the close-up shot in the first place.
Continuity mistake: When the garbage man is in Jameson's office, and Jameson is holding the Spider-Man suit, he is holding it almost at arms length to his right, yet in the following shot, it's a lot closer to his body. (01:05:50)
Continuity mistake: When the garbage man enters Jameson's office, Jameson is holding a phone to his chest, yet in the following shot he is not holding the phone, and we didn't see him put it down in the previous shot. (01:05:40)
Continuity mistake: When the people are watching Doc Ock with his experiment, Rosie is standing at the bottom of the stairs. When Doc Ock passes her his coat, she is then seen in the background standing next to a large TV screen. (00:32:35)
Continuity mistake: When Peter starts to lose his powers, and tries to climb up the side of the building, in several shots, he is by a window, and he slips opposite the window, yet in the following shot he is a lot further above the window. His distance and position from the window changes in subsequent shots. (00:56:30)
Continuity mistake: When we see MJ and her fiancee in her room close to the end, MJ says to him "Lean your head back for me." Her hand is not in his hand in one shot, but is in another. (01:23:40)
Continuity mistake: When Doc Ock escapes from the hospital, he hits a taxi with his tentacle. The taxi goes flying and is about to land on its roof in the middle of the street. The next shot however shows the taxi landing on top of another one, and the taxi is right side up.
Continuity mistake: After Doc Ock throws the car through the side of the deli, it shows Peter and MJ from outside the building and there's a table with a chair at the base of the hole right up next to the nonexistent wall. For the rest of the scene, the table and chair are not there.
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.