Continuity mistake: When Phil is telling Geoffrey about his day at work he takes an aspirin and drinks the water. The glass of water is almost gone. But if you look closely a couple of shots later the glass is full of water again.
Fresh Prince After Dark - S4-E9
Visible crew/equipment: In the episode where Hilary poses for Playboy, Hilary's strap-on mic is visible on the back of her red dress when she, Will, and Carlton first enter the Playboy party. It is gone in subsequent scenes.
The Fresh Prince Project - S1-E1
Revealing mistake: When Will screams in front of the mirror, imagining himself turned into Carlton, something went wrong making the composite shot and not only you can actually see the line of separation between the two frames, but the superimposed one exposes a bit of the mirror woodframe before it wiggles into the correct position and covers it up. (00:11:30)
Continuity mistake: When the family kisses Will goodbye and when he sits bummed out on the couch, the pool float around his arm switches position, going under his left arm, then under his right, then left again. (00:02:50)
Clubba Hubba - S1-E3
Continuity mistake: As Carlton and Will battle out with their "I bet he won't", "I bet he will", Will is holding his drink with his left hand. When he breaks the fourth wall with a joke in a new shot, the glass switches hands. (00:05:00)
Answer: He's coming back from his vacation in Philadelphia.