This happened in 2002.
I was allocated the middle seat of 3. On boarding I found both seats either side of me already occupied by 2 grotesquely fat Indian ladies and who had appropriate a part of my seat each and in order to fasten the seat belts had to use the belt from my seat. I protested and was met by a torrent of abuse in Hindi. I spoke to the purser and asked if there was a seat vacant in business class . No! she said forcefully ,the flight is full, you will have to get off. So I did, and complained to the ground staff. They went into a huddle, called a supervisor, who took me away to an office and went to investigate. In the meantime the flight departed with my baggage on board.
The supervisor came back and told me that the purser was incorrect, there were spare seats in Business and that the captain was perturbed that he was flying with baggage on board but without the passenger. He apparently had known nothing about my having to get off.
The airline fed me and got me on to their next flight which left 8 hours later. Unfortunately my request for an upgrade was denied, and I had to fly as self-loading cargo.