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.
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.