The Winner's Curse
You won the auction. That's the bad news.
What it actually is
Picture an item worth the same to everyone but nobody knows the true number, so each bidder guesses. Some guess high, some guess low, and the auction hands the prize to whoever guessed highest. That is the trap: winning is not evidence that you were smart, it is evidence that you were the most wrong in the optimistic direction. In a common-value auction the winning bid is drawn from the top tail of everyone's errors, so the average winner overpays even when the average guess was dead on. The fix is counterintuitive and painful: before you bid, ask what it means if you actually win, then shade your number down for the bad news you are about to receive.
Where you'll see it
In the 1970s, oil companies bidding on Gulf of Mexico drilling tracts kept winning leases and then losing money, because the firm that won was reliably the one whose geologists guessed the reserves were biggest. Three Atlantic Richfield petroleum engineers, Ed Capen, Bob Clapp, and Bill Campbell, wrote it up in a 1971 Journal of Petroleum Technology paper and coined the term. The same pattern shows up in corporate takeovers: the acquirer who wins the bidding war is usually the one who most overestimated the synergies, which is why the majority of acquisitions destroy value for the buyer.
Spot it in yourself
If you feel a rush of triumph the instant you win a bidding war, a hot job offer, or a contested house, notice that everyone with better information just declined to pay what you paid.
The bias underneath
The curse is not that you were an optimist, it is that you ignored the most diagnostic fact available: winning the auction is itself evidence that your estimate was the highest, and therefore probably the most inflated.