I was playing with friend on Halo 2: Anniversary online campaign coop when we both went to use a terminal. We were on legendary and one of the enemies was still alive, he killed one of us while we were in the terminal and when we got back to the mission everything was desynced. I was still doing what I was doing on my screen but on his screen I was doing it but in a different area and vice versa. When I was moving around on his screen I would jump off cliffs and restarting the checkpoint for him and vice versa. When I would die on his screen though I wouldn’t reset. We tried restarting the mission and it worked until we died again then the issue came back.
Edit: He also made it to a cut scene in the mission while I was still in the previous area.