In the first bar scene, Popeye Doyle eats an egg that changes from partially eaten to whole again and back again while he tries to talk to the French girls.
During the first bar scene, Popeye Doyle's drink goes from a small shot to a nearly full glass and back again during his attempted conversation with the French bartender.
In the beginning of the movie, when Doyle arrives with his suitcases at the dock, a girl in a flowered dress and a boy in a yellow shirt run past him towards his right-hand side. In the next shot, when we see Doyle from the front, the same girl and boy are climbing up on a fence on his left-hand side.
After Doyle is dumped in front of the police station and the doctors and police are working on him, and administering first aid, you see a doctor intubate him with a white tube, when the doctor later removes the tube, it has been shortened to a half-inch from the 3-4 inches from when he was originally intubated.
Towards the end when Jacques smashes through the front windshield of the van, the glass breaks into large shards instead of safety glass which is used for vehicles.
Mayonnaise originates in France and is a very popular condiment in that country, it is unlikely that a French policeman would be clueless as to what Popeye is talking about when he asks for it on his sandwich.
During the final sequence into which Popeye Doyle chases Charnier on foot, he scrambles through a bus depot trying to locate him. The reflection of the cameraman can be seen on a Brandt advertisement as he searches one bus.