It is the fault of both. Having done both I never got the wrong club in the bag, or played another players club. First if you lay your players club across another players bag to retrieve a divot, rake a trap or the get flag, and it does happen, you need to be on it enough to remember where you had put. If another caddie put their players club across you bag you should be aware enough to know it isn't your players. And finally as a player even if it is the exact same club most have characteristics that seperate them from other players clubs, ie markings, different grip, lie or grind. Two people not catching the same mistake? Someone hit the pubs the night before... Done that too!