This slip-up is in the scene where Val and Earl are preparing to ride to Bixby. Watch around the time when Chang comes out and says, " Here's some swiss cheese and some bullets." Always keep you're eyes on the horses, especially their rears. Either when the discussion about who gets which gun is happening, or before, a crew man walks from behind one of the horses rears to the next. He's carrying a boom mic, and I believe he is wearing sun glasses. It took me years of watching this movie over and over before I noticed this, but once I did, it's obvious.