Extreme version of this, to BeyondTheImpossible levels... When This Troper's mother was in high school, she told a friend a joke (She doesn't remember what the joke was now. Something about ferrets...), and the friend didn't get it, even though everyone else was cracking up. '''''Seven years later''''', they happened to be hanging out together, and the friend suddenly burst out laughing so hard she fell out of her chair. Turns out, she had
just gotten the joke, which had nothing to do with their present conversation.