Audio problem: When Cliff asks Rudy what's her boyfriend's name, you can hear a man off-camera saying "Bud" before Rudy answers.
Continuity mistake: After Cliff talks to Peter, Theo is told to run to the neighbors across the street. However, he can be seen in the background of some shots when the twins do their game. As soon as the twins finish, Theo is shown coming back into the house.
Continuity mistake: In this episode, Rudy is riding her bike around the house which is right on the corner of a block. Since when is their house on the corner?
Eat, Drink and Be Wary - S8-E16
Continuity mistake: This episode was obviously filmed before the kitchen was re done, but was aired after that.
Continuity mistake: Cliff says "a young trumpet player while sitting in the bandstand watching the dancers". But in all the later episodes, Russell plays the trombone, and he had been for a long time.
For Men Only - S8-E9
Factual error: When Olivia (or whoever) is teaching Russell how to play the Nintendo game she tells him the controls and includes "C and D". The old Nintendo system had no C or D.
Continuity mistake: When Cliff returns from the store for about the fourth time and the eggs and tomatoes fall out of wet bag onto the floor why aren't the eggs in a carton? The bottom of the bag drops out, so the carton can't be in the bag.
Revealing mistake: When Robert grabs Scott's shirt, Scott hits the wall and it moves backwards about an inch showing that it's fake.
Other mistake: It is daylight outside when Cliff and Clair are talking, but when he turns the lights off to try on his light-up Father's Day gifts, it gets pitch black in the room.
Continuity mistake: When Pam is in the office talking to Cliff and he tells her about the curfew time, she has a pencil in her hand and a pad in her lap. In the next shot, Cliff offers her a pencil and tells her to take the pad (which is on his desk).
Rudy's Walk on the Wild Side - S6-E18
Character mistake: At the beginning, when Vanessa is begging for money from her dad to buy a CD, Cliff says she can clean the garage. But the Huxtables live in a brownstone townhouse and brownstones do not have garages.
Denise Kendall: Babysitter - S6-E6
Continuity mistake: In the scene where Bob gives Elvin the apple cider and another jar, when you first see the two jars, the jar in his left hand has a few inches of foam on the top. Bob sets the jars on the table, and the foam is gone. When the shot returns to the table the foam is back again.
Other mistake: The outside shots show that the house is in the middle of a row of houses, connected to other houses on both sides. But in the living room, behind the stairs, is a window looking outside. There couldn't possibly be a window there as that is where the house shares a wall with the house next door.
Deliberate mistake: In the very first episode, the kids are fighting and arguing all throughout the house. Claire then asks Cliff, "Why did we have 4 kids?", referring to Denise, Theo, Rudy, and Vanessa. In all of the following episodes, they actually have 5 kids. I guess they decided to add Sandra (the oldest Huxtable) later down the road.
Revealing mistake: Everytime someone opens the front door to go outside there is just steps leading outside, but when they are on the steps outside and the doors to the inside are open there is a wall and a table with a lamp. Inside that is not there.
Continuity mistake: The father was known as Clifford in the beginning but the name was changed to Heathcliff (you can see this on the nameplate for his office).
And So We Commence (1) - S8-E24
Plot hole: When Theo is complaining that his father is inviting too many people to his graduation, he says "I saw him in the backyard 5 minutes ago hosing down the beach chairs", but this couldn't have been possible because in fact, Cliff was upstairs right before that scene, and then went straight to the living room, so there's no possible way that Cliff could have been in the yard at the time Theo had said he was.
Back to the Track, Jack - S1-E20
Character mistake: When the race ends and the kids are cheering, you can hear Denise call Cliff "Bill" instead of "Dad".
War Stories - S3-E11
Continuity mistake: Towards the end of the episode, Cliff Huxtable is sitting in the living room with his father Russell, Sgt. Stokes, and Homer. He is telling them of an incident that happened while he was in Bermuda. He is using dominoes to explain his story. At first there are about six or seven dominoes standing up on their sides. The camera angle switches, and all the dominoes are flat on the table. It goes on for a minute or so with the dominoes lying flat on the table. Then when Russell, Sgt. Stokes, and Homer get up to go for a walk, the dominoes are standing up again.
Mr. Fish (a.k.a. Goodbye, Mr. Fish) - S1-E2
Continuity mistake: While some of the family is eating, the cup in front of Denise disappears than reappears.
Chosen answer: That is the song "Open the Door Richard."