I absolutely detest when the author says that a character gives a witty remark or a scathing reply, but doesn't say what the character says.
Ah, the verbal equivalent of the "informed attribute".
"Jane replied to his ignorant comment with a witty putdown that had the entire room convulsed with hysteria" means "the author couldn't actually think up a good response, so just tells you it was a zinger".
On the other hand, that's not as bad as an author proving that no, s/he cannot write poetry, or come up with a good witty response even when given plenty of time to think about it. If someone is described as the greatest songwriter of her generation, including third-rate song lyrics does not in fact add verisimilitude. There are novelists who can also write poetry, and some of them include it in their fiction when appropriate. For example, Emma Bull is both a musician and a novelist, and her fantasy novel <cite>War for the Oaks</cite> includes original song lyrics, attributed to the protagonist, and they're good enough for it to work. (Nor does Emma claim to be the greatest songwriter of her generation.)