Replacing the Android home buttons with the swipe up gesture. It was demonstrably a very clear usability and efficiency loss, but most people strongly preferred it.

Before we had that latter data I actually argued against attempting it - I figured having a clear usability win vs iPhone would be an area we could capitalize on, and didn't believe we'd be able to execute the swipe system well in the time we had (I'd rather be behind and robust than leading edge and flaky), but doing it was definitely the right call - felt pretty sheepish about that one for a few years. The eng and ux teams that pulled it off were next level.