Ummm i would say mid-battle.

One time someone dropped while tanking boss.

Once i believe we just engaged some mobs, but the person could have d/c'd prior yet only noticed then. He was regen'n mp back *standing still*, im fairly sure.

Another time was while we where running. I believe not in combat that instance.

Not exactly sure if its an ability or what because it's not me that has been d/c'n....yet.