Similar situation to BrandeX: not in my GOG library, but I have a bunch of stuff from Humble Bundles that I redeemed on Steam and periodically see a game and go "When the hell did I get that?"

Oh yeah.

I never had that situation, unfortunately. (certainly because I've just bought 2 bundles so far and need hours to weigh the pros and cons of buying a game, reading through dozens of reviews...)
It must be funny to find a game you don't know of anymore.
The only time I was surprised is when the Larry Bundle split up and there was 'Softporn Adventure' in my library, that was quite strange...