Continuity mistake: During the train scene, Spider-Man's mask had gone partially black. We also see it when Spidey puts his mask back on. Yet when Doc brings him to Harry, we don't even see a patch of darkness on his mask.
Continuity mistake: When Dr. Octopus is robbing the bank, NYPD squad car 4619 arrives on the scene. However, squad car 4619 was badly damaged during the pizza delivery car chase earlier in the movie - an awfully quick fix. (00:24:55 - 00:49:45)
Continuity mistake: After Spider-Man recovers from the train incident, Doc Ock smashes in the other end of the train and throws the door on the floor in front of him. The camera cuts to another shot, and when it returns the door is no longer on the floor, nor is it there for any subsequent shot.
Continuity mistake: When Peter misses the class at the university, the way he holds his books/folders changes when the teacher talks to him.
Continuity mistake: Peter has a small horseshoe-shaped scar on his right cheek. In Dr. Octopus's lab, as Octopus is destroying the fusion reactor, they share a meaningful look and the scar has switched cheeks. (00:26:15 - 01:46:15)
Continuity mistake: When Aunt May hugs Peter near her "hero" speech, she hugs him with her head to his left shoulder. The shot changes and you see that she is now on Peter's right shoulder. The shot changes back and you see her on his left shoulder again.
Continuity mistake: When Aunt May is falling off the building, her purse is on her right arm. The next shot shows the purse on her left arm. (00:52:05)
Continuity mistake: Regarding the scene where Doc Ock has delivered Spider-Man to Harry's mansion, when Harry pulls off Spider-Man's mask, there is a very obvious and bad splice. The "mask on" Spidey's head is in a different position and further away than the "mask off" Spidey's head.
Continuity mistake: During the final conversation between Spider-Man and Doc Ock, the rips in Spider-Man's suit keep changing. For instance, there is a tear on his right shoulder; for most of the scene, there is a single piece of black webbing left holding the rip together, but when Doc Ock grabs Spider-Man's arm, the rip now has two pieces of black webbing. Then it goes back to one.
Continuity mistake: In the scene where Peter is riding his scooter to Mary Jane's play, after the music fades out you see a black convertible being chased by several police cars. The black convertible is rammed forcibly, and you see many hideous dents on the side of the car. Thirty seconds later, after Peter is done talking to the kids, we see the same car from the same side it got rammed into, but the hideous dents have vanished. (00:24:25)
Continuity mistake: As Peter stops the elevated train with many webs, a tear appears in his suit on the inside left arm during a closeup. Later, when his arms are still outstreched, there are many shots where you can see that the tear has disappeared; for example, a few shots later when he is still trying to stop the train; when the passengers are lifting him over their heads into the train; and many others.
Continuity mistake: In the scene with Peter's "I'm back!" jump, It shows a shot from above and you can see his solid outlined shadow in front of him. However, in the very next shot it shows you a close up of Peter walking, and there is no longer a shadow there.
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.
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)
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)
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)
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.