When the kids are first introduced to Basil's jazz band, hundreds of kids rush to the bar, with the main characters in the rear. Yet, the main characters somehow manage to find one of the few tables in the bar.
George Hamilton's hair is sharply parted, but the side of the part changes back and forth throughout the movie.
At the end, when Merritt sits down on the beach, there is a ship at sea in the upper left slowly going left to right. Two minutes later, when she and Ryder depart, the ship is missing. It was not moving fast enough to go across the entire seascape in that amount of time.
(at around 22 mins) As the girl is walking the "blind" man across the street, he takes off his glasses to reveal he isn't blind at all. The scene is shown from his viewpoint, which shows a red car coming through the intersection and just passing the girl. She gasps at him, then turns around, immediately walking through the intersection into the space the car should also be occupying at the same moment.
While Merritt is at the controls of Ryder's family yacht, only one throttle is forward on what is obviously a twin-engine boat. That would have made for considerable control difficulties trying to run the boat.
Scene at the beach shows a 48-star flag flying. By 1960, Alaska and Hawaii had been admitted to the union so a 50-star flag should be flying.
In the beginning, when Merritt is proving her point to the college professor, the audio doesn't match her words.