When in the woods and running from his 'captors,' Julien hides in the rocks. During this sequence, his hair changes between shots.
Jean's pants are dirty after wrestling with Julien, but clean at the restaurant.
When hiking, Julien asks what day it is and is told that it's Thursday, January 17th, 1944. That date was actually a Monday.
During the church scene, a girl looks directly into the camera.
Jean Bonnet is demonstrated to have surprisingly impressive pianist skills; However, when his hands are highlighted, his left hand is pressing the wrong keys, while the right one isn't moving at all and therefore it's clear that the music was added after the scene was shot.
During the bathhouse scene, one of the children on the left of the screen glances directly into the camera.