My BMW does this to. (yes 650 and the a/c works) I beilve it is because of the a/c raising the idle and giving the car more gas (black smoke rich) You may have shut the car of before the gas burned of and just had to much gas. IMO I have no clue why it would keep going. (BMW uses a idle control valve that ofeten will cause this problem)
Sorry I am not sure on these engines