Every damned time this happens. I get on a plane, and I have a bank of three seats, with no-one Sat with me. So I sit there, basically imagining how great it is gonna feel stretched out sleeping on these seats. Then the last person to get on the plane, usually just as the doors are closing, comes and sits next to me.
Addendum. When I got up just after this guy arrived, got himself comfortable, and then decided that he wanted the toilet, I noticed that the plane is nearly empty, so as soon as the seatbelt sign went out, I rushed back and found a spare bank of three.
Goodnight folks