Continuity mistake: When Tony walks into 2001 that first night, he shakes hands with some guy as he is walking through the club and again a few minutes later.
Continuity mistake: In the beginning sequence of the movie, as John Travolta is walking down the street, he looks at two girls whom he apparently thinks are cute. In the scene with the second girl, whom he stands in front of while attempting to make a pass at, the heavy-set female extra behind this girl, who is wearing a white shirt and carrying a bag with a tomato on it, first is walking behind the girl in the same direction she is going (coming toward Travolta), then in the next shot is seen walking away from Travolta and the girl, and then in the next shot is back to her original position and is walking behind the girl again.
Continuity mistake: Travolta's friends pick him up with the car to go to 2001 Odyssey. The roads are wet. When they reach the end of the block the roads are dry.
Continuity mistake: In the beginning, Travolta is walking down the street with a paint can. It's a sunny day. He then looks to cross the street, crosses, and now the streets are wet with rain.
Continuity mistake: The extra club fillers did not change their clothes in the disco scenes that occur on different nights, only Tony and the crew are wearing new outfits.
Deliberate mistake: The same group of people wearing the same clothes, including a dancer and bartender, are shown sitting around a bar on two different disco nights. On the first disco night we see them when Tony leaves the bar to go outside after being informed by Joey, what Double J is doing in the car. We see these people again at the start of the second disco night, both nights when we see them, the song "If I Can't Have You" is being played.
Continuity mistake: In the scene right after Bobby falls off the bridge, when the group is down at water level with the police searching for Bobby, Tony is seen with a Steri-Strip type of bandage and some sort of stitches covering the cut over his right eye (screen left). The cut goes from side to side. In the next scene, where Tony is on the subway, the cut is still visible, but there is no bandage nor stitches, and the cut is now vertical instead of horizontal. Also, he did not have the bandage nor stitches when he was on the bridge trying to console Bobby, or any time before that after the fight scene, at the hospital, or while dancing.
Audio problem: In the scene where Bobby is talking to Tony's brother (the ex-priest) in the nightclub about getting his girlfriend pregnant, his lips rarely move when he is speaking, when the scene is shot on his right side. His movements of his mouth do not match the dialogue.