The worst example of things popping up I've seen is Youtube's sponsored content warnings. These immediately appear above the active click target (video thumbnail) when you move your mouse over it, hijacking the expected action there. If you click things as a single action like every skilled mouse user does (rather than pointing and then clicking as two separate actions), it's physically impossible to react in time to avoid clicking them. And because only a part of the click target gets hijacked it's too inconsistent to learn to avoid it by intuition. I've clicked them accidentally several times and every time it's been disturbing because it feels like some serious and unexpected error.