I flew on UA741 the other night ORD-SFO on a 3-cabin 767 as a 1K - incidentally my first upgrade eligible flight since the computer system change. When I checked the mobile site shortly before departure, it showed that my upgrade had in fact cleared with a seat assignment but I ended up flying the flight in my original seat. When I walked by the seat the mobile site says I was assigned, it looked like someone had sat there. This isn't the end of the world for me or anything by any stretch, I had a exit row seat and I sat and worked for 4 hours, but, I'm not sure exactly how the accounting worked out being that all of the F/C cabin seats appeared to have been occupied. Has this happened to others?