Talking Chipmunks; I recently encountered one which had a live mag and the method for shutting down this particular aircraft was to turn both mags off. As you might expect the engine just ran on - the only way to shut it down was to turn off the fuel (NB this aircraft not fitted with ICO). Now, turning the mags off to shut down the engine seems like a good way of checking the the mags are dead in L, R and OFF configurations without causing a big bang in the exhaust system. The danger of a live mag is not from pilots who know the routine but spectators who inadvertently lean on the prop. On most other aircraft it's another good reason to leave mixture in ICO.