Winner's curse
The Winner's curse is a phenomenon that occurs in common value auctions with incomplete information. In such an auction, the goods being sold have a similar value for all bidders, but players are uncertain of this value when they bid. Each player independently estimates the value of the good before bidding.The winner of an auction is, of course, the bidder with the highest bid. When each bidder is estimating the good's value and bidding accordingly, that will probably be the bidder whose estimate was largest. But wait! If we assume that on average the bidders are estimating accurately, then the person whose bid is highest has almost certainly overestimated the good's value. Thus, a bidder who wins after bidding what they thought the good was worth has almost certainly overpaid.
More formally, this result is obtained using conditional probability. We are interested in a bidder's expected value from the auction (the value of the good minus the amount paid,) conditional on the assumption that the bidder won the auction. It turns out that for a bidder bidding their true estimate, this expected value is negative, meaning that on average the winning bidder is overpaying.
Savvy bidders will avoid the winner's curse by bid shading, or placing a bid that is below what they believe the good is worth. This may make it less likely that the bidder will win the auction, but it also protect them from overpaying in the cases where they do win. A savvy bidders knows that they don't want to win if it means they will pay more than a good is worth.
The winner's curse gets stronger as the number of bidders increases. This is because the more bidders there are, the more likely it is that some of them have greatly overestimated the good's value. In technical terms, the winner's expected estimate is the value of the first order statistic, which increases as the number of bidders increases.
Some types of auctions, in particular the Vickrey auction, can reduce, but not eliminate, the winner's curse.