Here's the other bookend--lots of people catalogued their great deals, finds and steals in an earlier thread. And it's fun to take credit for being lucky or vigilant.
But what about the opposite, where fate offered up a fantastic deal you turned your back on? Or when you learn something was special after the fact, and then realize "Gee, I helped Auntie carry one of those out to the dumpster!" We tend to block these unpleasant, regretful memories, but there is always at least one that lingers...
My biggest one was passing on a set of five Gaillard saucepans (with lids) with exceptionally rare *vertical* handles. I think the whole set went unsold on eBay for $150. Me so 'tupid.