While I was arranging a future trip to Portland (where my office is), I selected the exit row aisle seat (11C) on the Airbus A320 that would be used for that flight. You can do that (select exit row seating) when making reservations on a United Airlines flight if you are at a Premier Executive or higher level in their Mileage Plus program. I think this is one of the more valuable benefits of being a Premier Executive.
Anyway, later, when the trip was ticketed by our travel department, I went to look at my seats on United's web site and found:
Note that some crazy person had apparently selected the exit row middle seat right next to me, even though there were exit row window and aisle seats still available all over the place. What kind of nut was that. So, I changed my seat to 11D (the aisle seat on the other side of the plane) and went onto the next leg of my trip.
When I later returned to check on the seat, I found:
The idiot had followed me. I had a plane seat stalker...
After switching back and forth a few times and finding that the seat next to me kept being occupied by my stalker friend, I figured out that this is United's way of ensuring that somebody else doesn't sit in the seat next to me until and unless the flight is filled above some level of capacity.
I had heard that they do this, but this is the first time I had some reasonable amount of proof that it actually occurs (surprising that it took me more than a million miles to figure this out, isn't it). My understanding (rumor level only) is that this happens (on purpose) for 1Ks and above.
Of course, this is all probably moot since I'll probably be upgraded before I actually get on the plane :-) -- as evidenced by my Traytable pictures.