A man’s driving down the road behind an 18-wheeler. At every red light, the truck-driver gets out of his cab, runs back and bangs on the truck door. After seeing this at several sets of lights in a row, the car driver follows him until he pulls into a parking lot. When they’ve both come to a stop the truck driver once again jumps out, runs to the back and starts banging on the truck door.
The motorist goes up to him and says, "I don’t mean to be nosey but why do you keep banging on that door?"

"How long have you been driving without a tail light, buddy?"
demanded the policeman.

The driver jumped out, ran to the rear of his car, and gave a low
moan. His distress was so great that the cop was moved to ease
up on him a bit. "Aw, come now," he said, "you don't have to take
it so hard. It isn't that serious."

"It isn't?" cried the motorist. "What happened to my boat and
trailer?"