This sort of thing probably happens more frequently than I realise, but…
In this episode of That Mitchell and Webb Look, they make a joke about sentencing Jimmy Savile to three life sentences (we assume he did something bad). I guess it was funny in 2008 because he was widely regarded as a bit of a saint, hero and generally nice guy.
Now, of course, we know he was a serial child rapist - which basically ruins the joke.
I still find it funny though.