Revealing mistake: Apparently, Buck getting thrown off his horse, whose name was Rebel, wasn't in the script. When it happens, a startled Cameron Mitchell shouts at the horse using its real name, Prince. Despite the blooper, the shot wasn't cut. In fact, it happens at least two more times in other episodes of the series.
Deliberate mistake: At the end, the U.S. Cavalry "brand" Blue finds on the mule isn't a brand at all: it's obviously been applied with white paint to make it more visible to the camera. It's also unlikely that it would be so large, or that it would be located on the animal's neck instead of on its rump.