Like many others I bought Torchlight for $20 only to see it drop to $5 a few days later. It hurt but I felt better knowing that I supported the devs a bit more by giving them more money.
I also once bought a character pack for Marvel Ultimate Alliance for $10 after waiting for what felt like years for it to become the Deal of the Week. The next week it was dropped to $5 for DOTW. To this day I think that me buying it reminded Microsoft that it existed and they dropped the price. Or maybe it was just bad timing on my part. It still hurt though.