Baby (and several other women in the film, actually) sports an obviously '80s "big hair" perm, something that wasn't in-vogue in 1963. Many women during that era went for straight-hair updos or the "Jackie O" bob look.
When Johnny & Baby first dance with the other staff, Otis Redding's "Love Man" is playing. The film takes place in 1963 but Redding didn't record the song until 1967, shortly before his death. In fact, "Love Man" was recorded in the same session as "Sitting On The Dock Of The Bay" and wasn't a hit until the following year.
When Johnny jumps off the porch to fight the guy who's been taunting him, his belt buckle is buckled. Then, when he lands on the ground and starts fighting, his belt is buckled undone. Then after a punch is thrown it's buckled again.
When Johnny locks his keys in his car during a torrential rain, you can see the sun reflected shining brightly in the rear window of the car. You can also see that the ground beside the post he picks up is dry and sunlit and you can see shadows cast by the sunlight as Johnny picks up the post.
After Billy with the watermelons finds Baby in the staff area, they start up the long hill of stairs to the faint sounds of 'Do You Love Me' beginning to play. When they reach the the top in the next frame, presumably 2-3 minutes later, the song is still at the beginning.
Lisa decides to lose her virginity to Robbie. She walks to his cabin that night and when she knocks on the door, there is a white towel on the door knob, a warning that Robbie is "busy". Lisa opens the door and when she closes it, the white towel is gone.
Before the final show/dance, Baby is sitting on her bed getting ready. She is wearing a no-seam bra (not available at the time) and rolling up stockings before putting them on when her sister comes in. When Baby is at the party/final dance, she is wearing a backless dress (no bra) and when dancing, she is wearing pantyhose instead of the stockings (and necessary garter-belt) that she was preparing earlier.
When Johnny approaches Baby after he dances "Do You Love Me," he asks his cousin what Baby is doing there. She answers, "I carried a watermelon." In the next shot, while Johnny is walking away, she mouths the words but there's no sound. In the shot immediately after, she says the line again.
For the last dance, we see someone put on a record and "I've Had The Time Of My Life" starts to play. Yet, later, we see Kellerman talking with Tito, who is conducting his band, who are now playing the song.
When Johnny locks his keys in the car he breaks the rear window to open the passenger side door. We then see him go round to the drivers door and open it. Baby can actually be seen reaching across the seat and unlocking the door while Johnny is walks around the car.
In the last scene when Johnny and Baby start the dance, everyone in the audience is seated in a chair. However at the end of the scene there are no chairs on the floor. However, you see some of the male dancers stop dancing and begin moving all the chairs for everyone to join in dancing.
When Johnny has to bust the back window of his car, the shot inside the car shows that the entire window frame gives way, but from the outside shot, there is only a hole and the window frame is in place.
The film was set in the summer, yet the lake scene where they were practicing the lift was filmed in October. Since the leaves were already turning, they had to paint the leaves in the background. If you look closely you can see where they missed a few in one spot.