You start to tell a joke – and halfway through you realise you've forgotten the punchline.
It could happen to anybody.
But when it happens to a professional comedian it's never a laughing matter.
According to reports, Billy Connolly (70) struggled to remember some of his material during a recent show in the Waterfront Hall in Belfast.
"This is the weirdest show I've ever done ... " he's said to have told his audience. "This is awful. I can't remember what I was saying. I get wee gaps and just stop."
(I've removed a few Connollyesque adjectives from his reported comments since this is a family newspaper.)
Fortunately his Waterfront audience didn't seem too bothered. We're told they cheered and whistled their appreciation at Billy. And why shouldn't they?
The odd lapse hardly ruins an entire two hour, one-man stand-up show which, let's face it, has to be tough going for any performer let alone a man of gradually advancing years.
The problem is that, as even the amateur joke-teller will know, once you've lost the run of your story a degree of panic sets in which makes it that much harder to regroup your thoughts.
At least among friends you can come clean and confess you've forgotten your punchline.
At his sell-out show, Connolly was obviously among friends. One fan described to a newspaper how it was obvious from the Belfast audience that "they'd forgive him just about anything – and that includes a bit of memory loss".
He may forget the odd storyline. But I don't imagine Billy Connolly will forget that in a hurry.