A body of someone that has just drowned does not float on water. Only after the decaying process has started and gases build up in the body does the body float to the surface.
The Maserati Ghibli sports car is a V8 and should have 8 cylinders. Harry says it is a V8 with 4 cylinders when he tries to impress Jean-Paul at the end of his joyride.
When Marianne, Harry, and Fred are at the outdoor café at the harbor, there are dozens of people standing across the street watching the cast and crew film the scene. Some of them even take photos. (Scene begins at 1:01:02.)
When Jean-Paul meets his friend Harry's daughter for the first time he calls her by her name Pénélope in spite of the fact that, according to the full narrative of the film, he has never heard of her name before.
When Jean-Paul's and Marianne's telephone rings for the first time, she takes the call, and Jean-Paul ask who rang. Marianne tells him to guess. When he cannot, Marianne says it was Harry. Jean-Paul says that whenever they hole up somewhere, Harry is always the first to call them, why Jean-Paul should have known or be able to guess it. There is no explanation in the movie for this character error.