Three stand out in my mind and haunt me on a pretty regular basis:

When I worked at Wal-Mart from '96 - '97 and we had stacks of brand new Earthbounds and Virtual Boys on clearance for 20 bucks each.

I found a crapload of sealed NES games at a different Wal-Mart in Arkansas in the summer of '03. I bought most of them but left behind Color A Dinosaur.

A good buddy and I were big into flea markets and yard sales around this time. In the summer of '04 we went to a flea market in a building that used to be a car dealership. One booth had some NES games and I almost bought one loose cart I had never heard of or seen before just because I was looking for something new to play at the moment. I ended up putting it back in the box I had picked it from and leaving. It was Panic Restaurant