Continuity mistake: McQueen and Brynner take a hearse up the hill to deliver a coffin to the graveyard. When they go back down the hill, you can see a plume fall off one of the horses' heads, but in the next shot, the plume is back on.
Continuity mistake: in the scene where Horst Buchholz confronts the girl that was spying on him along the stream, her hair (braids) change position. When he first stops her and she begins to struggle and slapping him, he grabs her wrists. At first the shot is from behind her back and both braids are behind her. When the shot changes to face her over his shoulder one braid is on her chest.
Continuity mistake: During the hearse sequence, the duo passes the large red circular sign with the "5" in the center (first time code). They then progress to the General Store (the next-to-last two-story building) and Horst Buchholz waves them off. When Yul turns and says "I don't think so," they're passing the red nickel again (second time code).
Character mistake: Eli Wallach's character states a number of times he has 40 men. Assuming he is in addition to that, there should be 41, yet we see at least 44 die on screen. That does not count the ones shot at inside buildings when McQueen or Buchholz go charging through, or the one behind the window when McQueen is shot, or the ones that escape and ride away. This includes the 3 that spy on the town, 12 in the first shootout, and 29 in the big finale fight.
Continuity mistake: Late in the scene where Horst Buchholz's character confronts the girl that was spying on him along the stream, her hair (braids) change again. As she is responding "I will never tell you" her braid is on her chest. Then the camera changes angle slightly and the braid is over her shoulder on her back.
Visible crew/equipment: When the three Mexican farmers arrive in town you see the traces in the street from a car making a 3-point-turn in the front of the picture. You see the traces in later shots, too.