Unauthorized seat swaps were the custom in years past. On the 727 it was considered kind to put the FE in the FO's seat for a leg once in a while.
UAL had a DC-8 (double drop-rise) freighter crash after one of these seat swaps.
PROBABLE CAUSE: "The flight crew's failure to follow procedural checklist requirements and to detect and correct a mistrimmed stabiliser before the aircraft became uncontrollable. Contributing to the accident was the captain allowing the second officer, who was not qualified to act as a pilot, to occupy the seat of the first officer and to conduct the take-off."
http://aviation-safety.net/database/...?id=19830111-1