An occurance that indicates such stupidity or incomprehension that it causes a witness or participant to slap himself in the head in disgust, despair, or disbelief.

"He thought that all Canadians live in igloos. It was a real head-slapper."

"I found my glasses. They were on my face the whole time. What a head-slapper."

Try it. It works.

The MUD expression of this is "so-in-so bonks herself on the head". If you've ever done this (not necessarily in a mud), it is a Very Good Thing because it means you actually recognize and aknowledge when you've done something stupid, unlike most people on this planet.