When the random prisoner has his head cut off, if you watch it slowly, you can see the blade come down and stop, then the next cut (at the same angle) has a dummy's head rolling. Additionally, in the latter cut the Sun's position has changed, so it is now in the frame.
When the guards start to open the alligator's belly and shout "Move it! What the hell you looking at!" Dega is wearing his long sleeve dirty gray shirt with a wide brim hat (with nothing in his hands i.e., his striped shirt), and Henri is wearing a headband across his forehead. Moments later, when they walk towards a log to sit down, Dega is now wearing his orange and white striped shirt with a bandana type headpiece and Henri's headband is now wrapped and tied around his neck.
When Papillon is in his first solitary confinement, a vampire bat bites him and drinks his blood. The wound is two fang-like holes where the blood comes out. A vampire bat doesn't have fangs for teeth. Using their sharp teeth, the bats make tiny cuts in the skin. The bats' saliva contains a chemical that keeps the blood from clotting. The bats then lap up the blood that oozes from the wound.
The guillotine permanently erected in the prison courtyard is a piece of pure Hollywood. By the 1870's, the French had developed demountable guillotines that could be easily transported and set up in the town square wherever an execution was to take place, taking only an hour or so to assemble and dis-assemble. There is ample evidence that the 'bagne' (Caribbean penal colony) had one of these, which would have only been brought out of storage when needed. Had it been left permanently exposed to the tropical sun and rain, its timbers would have very rapidly deteriorated and warped to render it unusable.
In the last scene as Papillon jumps from the cliff after he throws the bag of coconuts, you can see in the upper left quarter of the frame what seems to be a camera helicopter shooting from another angle.