IIRC, he had no caption on the bottom (where it says "When Car Chasers Dream") and a firestorm broke out across the country...
People were outraged, this picture of a dog humping a car!
He was baffled, because that interpretation had never occurred to him.
He added the caption, but as he sadly pointed out, once you have to explain a joke, the damage is already done.
He said people were writing letters to syndicated newspapers demanding his perverted comic strip be banned...
(By the way, I read this from my books years ago and I might be remembering details incorrectly, but this was how I received it then!) I could just see all of this!