It happens more than I care to admit. One time that sticks out in my mind is that I showered, got myself ready, drove myself to work and suddenly became aware after I had been there for over 2 hours that I was even there. Apparently I functioned just fine but I don't remember driving to work and when I took my lunch break I discovered I had parked my van in the wrong place.
Since then I lose a few hours here and there. I've rather gotten used to it. It used to scare me though. Now I just get annoyed.