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.
Following the same theme of liminal UI areas, another thing that peeves me is touch screens like my local supermarket self-checkout which respond with a beep (indicating they know I hit the button) but do nothing, because apparently the beep-region and action-region and not the same. Or maybe I dragged my finger, in which case, please don't beep.
No one told me it had to be perfect, but I care about the user and don't ever want the user to be surprised.
That's the difference you get with outsourcing your engineering team vs developing in-house. If you choose a good team, your developers feel directly responsible for the user experience, go beyond stated requirements, and aren't just doing the bare minimum in order to make a paycheck. A good UX professional is constantly testing their work with unexpected input.