Mistakes and incoherences (various sources) :

Mistake – n° 1
In episode 120, Justin is wearing white pants when he's dancing on Babylon's stage. Later in the back room, he's wearing black ones.

Mistake – n° 2
In episode 119, Brian arrives at the diner, sits down and turns a cup over. When Justin arrives to give him some coffee, the cup is back to its original position.

Mistake – n° 3
In episode 107, Brian and Justin are dancing and you can see Emmett dancing behind them and crossing the screen from the right to the left. Then Michael and David arrive. When the camera focuses back on Brian and Justin, you can see the same shot of Emmett crossing the screen from the right to the left, they used the same shot.

Mistake – n° 4
In episode 101, while they're trying to give the baby a name, Lindsay says that Melanie's grandfather's name was Abraham. In episode 307, when they're thinking about a name for their second child, Melanie says her grandfathers were called Hyman and Herman.

Mistake – n° 5
In the credits of the first episodes, we can see that Chris Hobbs' firstname is Christian. However, in episode 201, the judge calls him Christopher... did he change his name in the meantime?

Mistake – n° 6
According to his birthday cake, Brian was born in 1971, and considering they're celebrating it just before the prom, we can suppose he was born in May. Debbie says in episode 213 that Michael was born in late March 1970. So Michael should be almost a year older than Brian. How come they both are celebrating their thirtieth birthday during season one then? (episode 111 for Michael and 122 for Brian)