The back button jumps past every video you watched

You work your way through the feed, one video after another, and the address at the top keeps up: every clip you land on gets its own link you could copy or come back to later. Then you press your browser back button, expecting the video just before this one, and instead you are back where you first walked in, usually the home page. Five or six videos gone in a single press.

The address changes as you move, but your browser is never told that going to the next video is a step worth remembering, so as far as it knows you have been sitting on one page the whole time. Each video becomes a step of its own, so back walks you through the clips you actually watched, one at a time, and forward brings you along again.