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.