I had that happen to me once. I had a semi-restored jukebox that a friend of a friend heard about via word of mouth. Said jukebox wasn't for sale, as I still had some bugs to work out on it, but when I crossed paths with these two at a show, I was asked, "How much for your 1448?" I told them it wasn't for sale, but you know the drill, and they kept pushing, so I "upped" the price in my head and said, "$8,000", thinking this would shut them up.
Nope. Guy pulls out a wad of cash and starts counting it out, so I had to "put up or shut up".
And I did....while laughing all the way to the bank!