moving mouse targets. Like let's say you have two pinned items on the start menu, Firefox and steam. You click Firefox and it starts to open. You go to click steam, but Firefox finishes opening and the icon gets bigger. Steam's icon then moves to the right, so you click where it was but instead just hit Firefox again. It's stupid.

Note how Firefox has solved this with tabs. Open a bunch of browser tabs. Enough so they shrink a little. Then rapidly close some, starting from the left. Notice how they don't change size until you're done closing tabs.

Mouse tunnels. Like you click the "File" menu, and then mouse over "New" and a long sub menu opens. Longer than the original File menu. If you mouse directly from the top of File to the bottom of New, your cursor will briefly be outside either menu. This often will cause the entire menu to close. Mouse tunnel. Have to keep the cursor in the tunnel. Annoying.

Had an old job that insisted this was fine and refused to let me or anyone change the interface to fix it (on a website)

Focus stealing. Like you're typing, and some other application pops up and takes focus. The absolute worst is when it pops up and puts focus on a dialogue box, and you just happened to hit "enter". Instead of adding a new line to your document, you just accepted something. Awful.