Gyllenhaal chipped a tooth after whacking himself in the mouth with a rifle during a scene in which he puts a gun muzzle in a co-star's mouth.
He was so incensed he then began showering punches on the unsuspecting, unnamed actor.
Gyllenhaal tells gossip site The Scoop, "For some reason. I just got so angry that I had chipped my tooth and just started hitting him and we didn't talk for a month after that."
Director Sam Mendes explains the incident, "It's 115 degrees, you're away from trees, cars, everything. You can't hear anything because the wind's blowing. You don't have your clothes, your car, your girlfriend, anything. And I really think Jake forgot he was acting a lot of times."