When you are doing a show at a comedy club. When you are pulling a prank on April Fool's Day. When you say, "Have you heard the joke...." Probably you right now with this question.
Have you heard the joke about waiting in line and hell??? ;) ;)
There's no particular situation. Once in an odd while, I deliver a dose of irony. Those who know me well recognise it instantly. Those who don't might take it seriously. I don't mind either way, though I'll admit that when people do laugh it gives me a lot of pleasure.