Other mistake: When Will confronts his mother about not wanting to move back to Philadelphia, she mentions she can't afford to send him to an out of state school. However, in season 2 it's already been established that Uncle Phil is Will's legal guardian, and he's a resident of California. Will's mother has it backwards. Going to school in Pennsylvania would be an out of state school and staying in California would be in state.
The Fresh Prince of Bel-Air (1990)
1 other mistake in season 3
Starring: Will Smith, Karyn Parsons, Alfonso Ribeiro, James Avery
The Fresh Prince Project - S1-E1
Revealing mistake: When Will staples his Malcom X poster to the wall, the wall bends back and forth for a second or two.
Hilary: He's punishing her.
Philip: No, Hilary. I just want her to spend some time with her family.
Hilary: Same thing.
Papa's Got a Brand-New Excuse - S4-E24
Trivia: At the end of the episode, as the camera zooms in on the statue on the table, Karyn Parsons can be heard crying backstage.
Answer: He's coming back from his vacation in Philadelphia.