It's happened countless times, to the point I don't even recall most of them. I get this stupid notion of not needing something when I find a good deal, then leave it. My rational mind kicks in later and realize that I made dumb decision.
Happened the other day at GS. Saw a game on my wish-list, $12. Nah... maybe next time. I leave, "man, that was kinda stupid to not pick it up". Go back a few days later, it's the only one missing from the shelf. Thankfully, another GS had it too. One of those formerly NEW converted to PRE-OWNED type of deals, so they all had one.
I've had this paranoid delusion that anything I do pick up you guys can thank me, because the price will stay reasonable. If I don't ever get around to it, it gets mega expensive. I kinda never thought Gravity Rush Remastered would be a hard get, I didn't pick it up at launch and then it exploded. However, I DID pickup the sequel near launch. The value dropped, and stayed down on that one. You're welcome.
I think one time back in the early days I passed on a copy of Intelligent Cube, didn't know what it was. Don't really regret it still. Passed on a cheap copy of Kingfield long-box because no manual, don't really regret that. I got the jewel case version pretty cheap, but still those games are going for a fortune now. I guess there aren't any in particular that really haunt me.