When Geo checks the corridor with the mirror and takes a break to say goodbye to his cellmates, he leaves the mirror in the hole. However, when he goes back to checking the corridor he has to stick the mirror back into the hole first.
In the sewer when Roland Darban is sketching the prison plan, the dirt on the ground is totally flattened while it was a passageway and there should have been some footprints.
When the cave-in occurs and Geo is buried, Roland starts digging him out with a shirt on. However, in the next shot he is suddenly no longer wearing a shirt.
Monseigneur steals sand and two ampules so that the inmates can MacGyver an hourglass and accurately measure time. After they succeed and test the hourglass, the inmates state the exact time in minutes one turn of the hourglass measures, while previously stating that none of them had any watch or clock.
(at around 1h 9 mins) This is not a goof. Just as the sand runs out prison/church bells ring. They had adjusted the improvised hour glass and sand volume after the nearby turret clock which can clearly be heard chiming three times. It is fairly common to broadcast the time by also striking the quarters well as the hours. At the beginning of the film during the establishing shot over La Santé Prison church bells can be also heard striking for a prolonged time.
(at around 1h 9 mins) This is not a goof. Just as the sand runs out prison/church bells ring. They had adjusted the improvised hour glass and sand volume after the nearby turret clock which can clearly be heard chiming three times. It is fairly common to broadcast the time by also striking the quarters well as the hours. At the beginning of the film during the establishing shot over La Santé Prison church bells can be also heard striking for a prolonged time.