Years and years ago I found a copy of "TT Racer" by Digital Integration on Amstrad CPC 464 cassette. I didn't have enough money so went to get some and when I came back it was gone! Never saw it again until emulators!
The other would be when an arcade closed down and seemed to still have old games in it, including Namco's 3 screen "Driver's Eyes" (a version of Winning Run). I asked about if the owner wanted to sell anything, but said the whole lot had been sold on to someone else. I peered through the doors of the place a bit later and could see smashed up arcade games inside. A little while later the whole old arcade building burnt down, presumably taking the games with it. A shame.