Oh, are we talking about bad YouTube UX? How about the "feature" where the right and left arrows seek the video 5s forward and back, while the up and down arrows increase and decrease the volume? That is, unless the last UI element you've touched was the volume bar, in which case the side arrows will also change the volume, and you'll have to use the mouse to clear the focus away from that volume bar to be able to seek the video again. I still wonder how they managed to break this despite it having had a sane, consistent, defined behavior for probably over a decade before that point.