I suppose something like this happens from time to time. Put it down to human error. I imagine that had the flight not been full and a dispute over seating not occurred, the baggage of passengers on wrong plane would have been unaccompanied to the original destination.
Some years ago I was allocated a seat (13A) at check in, when I got to the departure lounge, (we were checked into the lounge rather than through the gate) the staff took my boarding pass and issued me with another nominating a new seat. "We are not using row 13 this flight." I was told. I duly sat in my new seat and soon it was apparent that there was something amiss. No less than four members of the crew did a head count. Much scratching of heads, finally I was called by name to identify myself, which I did. I was asked quite agressively "Why are you not sitting in your allocated seat?" I explained that I was and produce my BP stub. Silence. I was left with the feeling that the delay was my fault, and the in-flight service reflected that to the destination.
D.