The primary purpose of a graphic is to inform. A design can also entertain or persuade. The essence of successful graphic design is communication. A graphic is successful when the message is received quickly and efficiently.
Think of highway signs. There are two messages a highway sign puts out. The notion of proximity and what it is that is close to you. Different signs mean different things and they are distinct. There is no one sign that delivers more than one kind of message. You don’t see signs that say rest area and speed limit all together. You see a group of signs that contains a message for this, a message for that, a message for this, and so on.
A graphic design fails when it’s message is disjoint or ambiguous.
There is some humor in this. The politically correct way to put is to say that the client is not reading the sign right. Unfortunately from a design standpoint, that is the ultimate fail.