I've passed up plenty of games only to find out later they were rare. But the only thing I REGRET is a Venture stand-up arcade machine at a local thrift store. This thing looked bran new. Clean, great side art, and it played like it was just uncrated. I have honestly never seen a cleaner system.
Asking price? $100. Reason why I didn't get it? I didn't want to bother my dad for his truck that weekend. Found out later he had been doing hauling for some friends that same weekend, and he could've picked it up and delivered it to my house that very afternoon. x_x Of course, the machine was gone next weekend.