Inspired by this post. Time and time again I come across witty insults like those attributed to famous historical figures; usually Winston Churchill. But the authenticity of them is always rather questionable, especially since often times they're nothing but a quick private exchange at a party or even in a bathroom.

So are they actual dialogues that happened between the two people quoted? Or are some of they legitimate, some of them made up? Or did they happen with someone, somewhere, and just got quoted to a different historical figure whose personality fit?