Ohhh yes, they keep slipping down at the most inopportune times. And as you mentioned, I forget to toss them. The other irritant is that pair of socks that for some reason, one wants to slip down into your shoe. Makes me want to throw a shoe across the room.
And why is it that if this happens (underpants or socks) while at work that is all you can think about! I hate that.