When starting a game with the options as: You----------------Opponent not AI, MP party. not AI not MP party. The players get swapped, making the opponent use his MP party.
I hit this yesterday or the day before on the test server, was trying to run monsters on one side vs. MP party on the other, and who was the monsters each time was wrong. But I was running two accounts against themselves and thought it was human error. EDIT: To be clear, mine was: Not AI, Not MP party vs. MP party.