Windows updates and other updates that decide to update when I'm right in the middle of something.
Windows 10 has been trying to foist this "upgrade" down my throat for some time now. It will just start running and then take hours to install. At the end of the install...it doesn't.
So I get a screen note saying "Restoring your previous version of Windows". (I already have Windows 10)
This has happened three or four times now and it never has actually worked once.
Sometimes it freezes up my computer so the only way I can turn it off is to unplug it.

!!!*$#$&&!!!