A particular bane of mine is the self-oscillating UIs, youtube is particularly bad at it these days - if the mouse pointer is in 'just the right place' (which is bigger than it sounds) then you get the seek-bar preview frames popup, which moves things just enough that the mouse pointer is no longer over the area that triggers it, so it vanishes, and the whole thing starts again.
YouTube also has the problem that there is almost no dead space to "park" your cursor. You have to carefully place it in between autoplaying previews.
This gets doubly frustrating when navigational elements don't contain actual anchor links. If you long-tap on mobile to open in a new tab, the browser decides you are selecting text and there is almost no way to dismiss the resulting copy/etc popup without tapping elsewhere on the page - which you cannot do without navigating elsewhere or otherwise causing an unwanted action.