I study the role of minimum bid increments (MBI) in internet auctions using natural field experiment data from a Finnish online auction site Huuto.net. Internet auctions are typically viewed as second-price, implying truthful bidding. However, due to the presence of the MBI, equilibrium bidding behavior involves bid shading. I sell otherwise identical gift cards while varying the MBI and test between truthful bidding and bid shading. Truthful bidding is rejected. Consistently with equilibrium behavior, bid shading increases as the MBI increases. This implies that real-world bidders are capable of reacting rationally even to seemingly small variations in their strategic environment.