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. (00:03:15)
Continuity mistake: When John Travolta and Karen Gorney leave the dance club after giving back their first prize for the dance contest they head for the car. Before they get in the rear window on the passenger side is closed. After they get in it is open. After Travolta gets out it is closed again. (01:42:30)
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. (01:49:30)
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. (01:04:45)
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. (00:21:20 - 00:56:30)mightymick
Trivia: When Tony Manero's father slaps him on the back of the head during an argument at the dinner table, Tony replies, "Would you just watch the hair? You know, I work on my hair a long time and you hit it! He hits my hair!" John Travolta ad-libbed this line. Director John Badham thought it was so perfect for the character of Tony Manero that he decided to leave it in the film.