A funny blooper that is not in a John Wayne movie is one I spotted in 'The Naked Spur'.
After Robert Ryan's character guns Jessie down, Ryan and his on screen girlfriend move to the top of the mountain to watch for the oncoming Stewart. While Ryan is surveying the terrain for his vantage point Jessie's mule comes over and licks Jessie on the face, the character who plays Jessie moves his face quite abruptly, he was obviously both startled and surprised by the actions of the mule.