On a recent trip, I missed my connection flight. But UA rebooked me on a late flight without a seat assignment. There was no seat available on the seat map at that time. I asked a UC agent and she assured me that I had a seat on this late flight. After an hour or so, the computer assigned me a seat. There were a couple of seats open later and I was able to move to an isle seat. The flight left with about 5 empty seats.