Spacing out
I space out a lot. It often happens when people are talking to me, which is annoying, because then I have no idea what's just been said and must employ the "nodding and ummming" technique. Sometimes, I'll know how important it is to listen (instructions are being given etc) and I'll remind myself to concentrate, yet the spacing out still occurs.
Oh, and when I space out, a lot of the time, I'll be thinking about something random and it's as though I can't stop my mind from wandering. I bet this is normal :P