Focus stealing - it winds me up more than is probably reasonable.

I'm there, typing away in my text entry UI widget of choice, purposefully entering text to do some thing as expediently as possible and then... oh, well I'm not any more, am I?

No, because some other window has decided that it needs the keyboard focus for usually no reason whatsoever, other than poor programming manners. Sometimes the same app will keep stealing the focus - oi, mate, I was typing over here? Could you not get the current focus and see that I'm busy?

This happens in chrome's dev tools _a lot_ but dev tools needs a rant all of its own.