Revealing mistake: In a scene where the canoes are shooting the rapids through a sheer cliff you can clearly see a stuntman climbing out of the water onto the rock.
Revealing mistake: After Jon Voight has killed the mountain man and has tied a rock to his body so he can dump him into the river, the "dead" man takes at least two deep breaths right before he is rolled off the rock and into the water. In that same shot where you see his chest heave and him noticeably breathing, he also flinches and moves the pinky finger of his left hand.
Revealing mistake: In the famous "dueling banjos" scene I noticed several errors revealing that the actors are not really playing their instruments. This happens throughout the whole scene. The banjo player is using a different picking technique than would be necessary to create the music heard. His finger patterns on the fretboard don't match the sounds heard. The guitar player strums when you hear individually picked strings and visa versa. Again, his fretboard fingering does not match the music heard.
Revealing mistake: The entire sequence of Voight climbing the cliff face is shot day for night, meaning the scene is supposed to take place during the night but was shot during the day. This is painfully obvious by the amount of contrast in the scenes we see sky (and the colored shots when he is on top of the cliff). When the camera pans up, it is a blue sky day but the scene takes place at night.
Revealing mistake: When Voight goes over the waterfall just after his canoe breaks in two, it is a stuntman.