Continuity mistake: In the scene where Will tells Vivian and Phil that Carlton is in Compton, Vivian takes off her earring from her right ear. A few shots of Will pass and when she goes to take off her other earring, the left ear doesn't have one, and the earring is on the right ear again.
Plot hole: Ashley says she is 9; later in that season, in "Just Infatuation," Phil says she is almost 12. Will is 17 for the first two seasons, and 18 in season three. Nicky Banks grows from baby to preschooler between seasons four and five (though this was humorously addressed).
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.
Continuity mistake: A member of the poetry club responds to Will's "You're a poet and you don't know it!" with a sassy "I -AM - a poet and I -DO - know it" in a close-up. In said close-up, she is holding her hand on a book, while in the shots before and after, her arm is in a different position. (00:06:30)
Continuity mistake: During the award dinner, when Hattie says the line about "Zeke sprung from the ground like a big ol eggplant," watch the table in the back, next to the door. The man sitting there wipes his mouth raising a napkin 3 times in 3 separate shots. (00:19:00)
Character mistake: When Geoffrey serves tea after his sassy cough at Vivian's sentence about High Tea not being a sacred institution, the actor messes up when it's time to actually serve tea, with a napkin getting caught in the teacup saucer. Granted, accidents do happen, but it's quite uncharacteristic for the perfect butler Geoffrey is portrayed to be, and simply a flub by Joseph Marcell. (00:17:45)
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 Will puts the last of his wall-shaking staples on the Malcolm X poster before the butler's interruption, the poster is almost perfectly flat. When Geoffrey walks to the poster, a bottom corner is noticeably raised. When Carlton walks to it, it's flat again, and raised when Will freaks out at the mirror. (00:07:40 - 00:10:00)