A few weeks ago, my wife and I selected row 1 A and B, bulkhead, on an Oasis. We took off, and after an hour or so the captain announced that we had to return because his and the first officer's airspeed indicators were giving different readings. We had to deplane until it was determined the problem could be fixed, or if another airplane would be required. A couple of hours later, we boarded a substitute plane. As my wife and I entered, we saw that another couple had taken the bulkhead seats--they had been assigned 3 A and B and wanted to stay where they were.

FA quickly intervened with a general announcement that everyone was to sit in their original seats, ignoring the seat numbers.