When the now-bandaged Maxence shows Mathieu around on his first day he shows him a wall with photos of the school benefactresses and the motto "Labor improdus omnia vincit" which is attributed to Virgil. But the second word is incorrect - it should read "improbus". Translations vary but it means something like "Persistent hard work conquers everything."
In the scene at the end of the film when the bus leaves (at around 1h 30 mins) you can see clearly the speed limit label on the rear of the bus (85 km/h). When the bus leaves for the second time it has disappeared (at around 1h 30 mins).
While Mathieu is auditioning more than thirty boys in his class to sort them by the pitches of their voices, discontinuities can be perceived in the relative positions of some of the boys. While Pépinot is walking to the left side of the frame after his audition, Boniface and one of the oldest boys can be seen standing together on the lowest step of the stairs in the center of the frame (at 33:43 to 33:45). The scene then cuts to a close-up of Mathieu calling out Boniface's name to sing next (at 33:46 to 33:48). Then the scene cuts to a medium shot of Boniface stepping off the stairs, but the older boy is not beside him (at around 48 mins). In a wider shot (at around 56 mins), the older boy is at the far right of the frame with the baritones and bases having had his audition "off-camera".
As Rachin enters the car and leaves with his family towards the end of the movie, crew members are reflected on the car windows. A man with a cigarette in his mouth can be seen (at around 1h 30 mins) reflected first in the front side window and then in the back side window as the car drives past the camera.
During the auditions, Mathieu sends pupils to the right or to the left showing the direction by his hand. When directing Ricoeur, who sings "I've got tobacco in my pouch", to the left (at 33:06 to 33:07) he first moves his hand to his right, which some cite as an error while others cite it as a flourish, but then sweeps or hooks his hand left.