Crew or equipment visible: In the very last scene as Papillon floats away there is a diver obviously under the bag he's floating on.
Audio/visual unsynchronized: When Papillon first visits the lepers, the leper he speaks to does not move his lips (probably because of makeup).
Crew or equipment visible: A harness is clearly visible on Papillon's midsection as he helps Dega over the wall.
Revealing mistakes: When Papillon is in the natives' village he goes swimming with one of the girls who has tan lines from a bra/bikini top.
Revealing mistakes: Newer prints, with improved resolution and brighter tone reveal that the leader of the leper colony's mouth isn't moving at the end of the "cigar" conversation, when he talks to McQueen.
Revealing mistakes: The alligator the two are supposed to grab has its mouth tied shut.
Factual errors: 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.
Crew or equipment visible: 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.
Factual errors: The guillotine permanently erected in the prison courtyard is a piece of pure Hollywood. By the 1870s 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.