Whenever M. Arpel parks his car in his tiny home garage, he always pulls in front-end first; however, whenever he leaves for work in the morning, the car always exits the garage front-end first. (This may be a subtle sight gag on (Jacques Tati)'s part.)
Twice in the film, when Hulot gets home, he opens the windows; he closes the right and leaves the left ajar, but in the following cuts, it is the opposite. (This may be intentional.)
When M. Arpal and his son are about to get into the car in the last few minutes of the film, the clock on the factory reads 3.42. As they drive off, it's 4.45.
A woman tells her guy that someone jumped in the river. He dives into the water and "rescues" a bunch of deformed extruded plastic. After he gets out, he chases M. Hulot and friends down the road. His clothes are dry when he catches up to them.
When interviewing for the job, Hulot's shoes are saturated enough to leave very distinct marks on the floor, chair, and desk, yet they leave no mark whatsoever on the floor after he puts his shoes back on.
When the boys are playing their traffic prank, one of their victims steps out of a 1955 Pontiac Chieftain. In the next cut, as he goes to argue with the woman he thinks has rear-ended him, the Pontiac is replaced by the 1951 Oldsmobile 88 driven by Charles Arpel near the start of the film (and which he is still driving at this point). Even the license plate (523 AP 75) is the same.
When Hulot rides his bicycle through the puddle and splashes water on a man, the water sprays from a hose hidden in the puddle and is not caused by Hulot's bicycle.
At the restaurant, the sounds of neither the violin nor the piano match what the musicians are doing.
During the garden party, when Pichard has just fixed the fountain and Hulot and Gerard are hanging from the round windows, a man in black clothes can be seen walking on the roof of the house.