One thing that instantly ruins my day is animation where the element state is inconsistent during the animation.

For example - I think it's on Windows where I keep running into this - on notification popups from some apps, you can't click "X" to close until sufficient time has passed after the popup animation. If you try to close it in that time, it instead acts as if you clicked the notification to open it. Super annoying.

Another, on Mac, when switching desktops, sometimes an app will show on the screen briefly before flashing to another app. The animation is slow enough that it often tricks me into thinking it's ok to do stuff and then nope the app changes. Even worse, I think it occurs randomly.

Note to designers: if you do choose to include an animation, it better be the most rock solid implementation ever or I will immediately think you're a twat and your app is a buggy pile of crap.