Character mistake: When Wes is drinking the mezcal with the worm in it, he makes a toast to "la vida luna" and tells Sissy it's Spanish for "the crazy life". This is untrue, the correct Spanish translation for "the crazy life" is "la vida loca". "La vida luna" literally translates as "the life moon" or better yet "the moon life", but a more accurate translation is "the night life."
Plot hole: Kane and Irons abandon their horses to hook a ride on the train, leaving behind their rifles. When they get to Hadleyville, Kane has somehow reacquired his Sharps.
Continuity mistake: When Cole and the halfbreed duel with knives, the fabric they hold in their mouths as a tether keeps flipping over, meaning the end of the cloth hangs out the left side of the mouth, then the right, and left again - several times for both actors - even tho the fight is continuous and they make no moves that would change this.
Factual error: There appears to be a pickup truck parked by a barn in the background when Steve McQueen is standing by his horse, talking to Linda Evans. (00:19:00 - 00:20:00)