The Hamburger Menu Doesn't Work
I'm surprised no-one has mentioned Android version 2 (vintage), which had a hamburger menu as a physical/soft button. It was then ditched and replaced with a app switcher button.<p>That hard button was even better from a real estate point of view, and since it was consistent across all apps it seems like users ought to have grasped it.<p>The surprising and important thing here is that, even if the user knows the menu is there and that, if asked, it could help them, it doesn't appear salient and doesn't get clicked.