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 - chronological order
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.
Vivian: Philip, when I met you, you were into James Brown.
Will: He liked James Brown?
Vivian: Even wore his hair like him.
Will: He had hair?
Trivia: Janet Hubert plays Vivian Banks, mother of Hilary Banks (Karyn Parsons). In real life, Hubert is only 10 years older than Parsons.
Answer: He's coming back from his vacation in Philadelphia.