Dropbuttons
One control with several answers, in every setting it is used in. A dropbutton is the only widget in the back office that has to open over the page rather than inside it, so the first thing this page is for is proving that it does: open any of them and nothing below it moves. The operations column of a listing twice, because a listing of config entities and a listing of content are not the same table -- one is short with identical rows, the other is long with rows whose actions depend on their state. The actions of a form twice, because a split button of links and a split button of submit buttons are built differently and only the first can be a links list. And the views UI, which is the heaviest user of dropbuttons in core and restyles them on top of whatever the theme did. Nothing is saved and every row is made up, so the page renders the same on every site.