It’s when you start a joke, perhaps one that the other(s) recognise. The punchline however, is not a subversion of expectation, which because the other(s) expect a subversion is a subversion in itself. Because the subversion is subverted the anti-joke is funny, even though it is terribly unfunny as a joke.
Yeah, now it says there’s 45 comments but I can only see 3