Basically, when odd conditions prompts someone to say something utterly crazy-sounding, and someone else (usually the local Deadpan Snarker) comments that "I doubt that's ever been said before" or "Now there's a sentence that doesn't get used much", or similar. Essentially a Stock Phrase, but hard to name as such since it can be formulated in a ton of different ways. I don't do this often, but I'll have to call a Seen It a Million Times on this - I can't think of any specific examples, but I know I've seen it all over the place. Ah, but I did manage to steal an example from another YKTTW:
Susan: But I want you to remember, I intend this breast satirically.
Patrick: Now there's a sentence that can't come up too often.