Have you ever played the "Spot the Difference" game, where you try to find slight differences between two similar images? I play it whenever I review code in a nontrivial pull request that changes a website's HTML, CSS, or JavaScript. Sometimes I win. Other times, I lose. But unlike the original game, where the player tries to identify a known number of harmless differences, sometimes it's difficult to determine the number of visual changes a code change causes. And failing to spot these differe...