It happened to me once. Shooting trap with other shooters who were knowledgeable, someone was putting my Browning Golden Clays Citori in their case as I happened to notice, leaving their Browning Ultra behind. Apparently he had just shot a round very well with it without noticing it. Being a regular shooter, I just thought it a mistake. Shooting sporting clays, the Churchill resides in the crook of my arm! At big sxs shoots I imagine that mix ups happen, but I try to prevent it.
Outright theft is an entirely different matter!
Karl