There were not enough uniforms for all of the stunt people and extras in the gun battle. If someone was filmed getting shot, the costume people would repair a uniform by washing off the fake blood, taping and painting over the bullet holes, drying the paint and sending either the same or a different performer out to get shot again.
This film was adapted from a story thought up by Roy N. Sickner, an actor and stuntman. Writer Walon Green wrote the script, which was then rewritten by Sam Peckinpah. Green felt that Peckinpah's rewrite was substantial enough to deserve credit, but Green had to lobby the writer's guild to allow Peckinpah a credit. Green has always said he was grateful to Peckinpah for not rewriting too much of the script just to get credit. Green, Sickner and Peckinpah all shared Academy award nominations for best screenplay (the only Oscar nomination Peckinpah ever received in his entire career.) They didn't win.
When Warren Oates emerges from under the bridge right before the Wild bunch begin their assault on the train, a large penis has clearly been drawn on the bridge to the left of where he enters.
WILHELM SCREAM: During the post office escape in the beginning, when one of the horsemen is shot in the face.