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.
A general golden UX rule is to never cause layout shifts on hover events, preventing this sort of thrashing. Unfortunately, Google/YouTube and others have lost the plot and no longer employ good UX professionals in their flagship products.
This was such a common thing to test for in fluid layouts under AIR/AS3/Flex, or any previous lingua franca where you had to design and code your own rollover effects or web components. So much is elided in mouseout and mouseover. Really, unless you've coded those events in a stack from movements, you don't understand what they mean. In any UI, it's the #1 thing you have to watch out for (along with stuck hovers, accidental focus and things like that). Mouseover/out depend on the entire screen graph as well as timing to establish user intent. Thrashing the whole UI off that because you shift the layout is the worst example of how to do it wrong. Amateur hour.