The safety-net trio of Undo, History, and Revert protect the intrepid image editor from unexpected disasters. But these commands also let you toggle operations, compare before-and-after images, and move back and forth through time. In this week’s Photoshop Top 40 Countdown, Deke takes us through the features of each by modifying a document using a number of features we’ve learned in previous Top 40 episodes, and then using Undo, History, and Revert to un-modify the document as desired.