I can’t say I’ve ever found a gem when I wasn’t looking for games at all, but I have found some when I wasn’t expecting anything exciting.
Probably 10 years ago, I was in a kind’ve sketchy store in a flea market near Baltimore and found a booth piled up with old electronics and games. Was not a very nice looking place and had bootleg GBA games in a glass display case and a table of more modern games up front. I dug around in the back of the store and in one pile of wires and controllers spotted a loose copy of Dragon Warrior IV for the NES! The lady manning the booth didn’t know/care what it was and sold it to me for $1. One of my favorite finds!
I also later found a boxed copy of FFV GBA (not bootleg) and a CIB Paper Mario Thousand Year Door there. Not sure what I paid for those (probably $10 a piece or something).
Talk about luck. I’ve not found anything good like that besides some Ubisoft filler.
I’m pretty intentional about finding deals, so I don’t really stumble upon anything unless I’m hunting for games. One of my favorites though was a Gameboy advance SP 001 with Pokemon yellow, silver, and gold for $70 at a pawn shop. All the games were legit and the SP just needed it’s battery replaced.
