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.
Factual error: In this poorly-researched film ("based on a true story"), Tom Horn lugs around a government-issue, lever-action 45.60 caliber rifle and even claims that he prefers the 45.60 because the ammunition is plentiful wherever he goes. However, there is no historical or biographical record of Tom Horn carrying and using a 45.60 rifle. In point of fact, the real Tom Horn's weapon of choice was the lever-action 30.30 rifle (a common range weapon of the day, often known as a "brushbuster"). When Tom Horn was arrested for murder in real life, he was carrying his 30.30 rifle, but a 45.60 cartridge was discovered in his pocket. This film fabricated its fictional plot around that 45.60 cartridge found in his possession.
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.