This paper presents a puzzle in the behavior of experimental subjects in what we call common-probability auctions. In common-value auctions, uncertainty is defined over values, while in common-probability auctions, uncertainty is defined over probabilities. We find that in contrast to the substantial overbidding found in common-value auctions, bidding in strategically equivalent common-probability auctions is consistent with Nash equilibrium. To explain our results, we run treatments to identify whether this difference stems from the way subjects estimate the good's value in a competitive environment rather than the way they bid conditional on these valuations. We conclude it is the former.