Audio problem: The Rabbi's horse is bucking in reaction to the rattlesnake, and all the while the Rabbi is yelling in Yiddish - but his mouth is not moving.
Visible crew/equipment: In a scene where Kirk Douglas appears to 'leap' onto his horse, a crew members arm is clearly visible in the foot of the shot pushing a mini trampoline next to the horse, ready for Kirk to jump onto and bounce up onto the saddle.
Factual error: The action in this movie takes place in 1869, yet half of the handguns are 1873 Colt Peacemakers, and the remainder that are Army or Navy Colts are all cartridge conversions, which were not available until 1871.