Well, sure, in practice someone who actually knows what they're doing also won't cause problems, even if they do happen to be the vehicle's owner. I assume thes

Well, sure, in practice someone who actually knows what they're doing also won't cause problems, even if they do happen to be the vehicle's owner. I assume these signs are so popular because just about every mechanic thinks they're funny, and they think they're funny because they have all seen bad versions of these kinds of customers, probably a lot, and likely far more than they see any kind of good version of these.