I had a similar experience as a staff instructor/tug pilot at Lasham.
We were pulling all the aircraft out of the hangar at the start of the day. The chief tug pilot was just getting in a Rallye 180 that need to be moved but he realised he had omitted to do a compression check. To save time I offered to turn the prop. We did the normal safety calls and responses. On the second swing the engine picked up at fast idle. I think he was more shocked than I as he immediately beckoned me around to inspect the engine controls whilst keeping his hands in the air. The tumbler switch controlling the mags was off with the key removed. The switch had failed to earth one or both magnetos.
Perhaps it helped that I had done my PPL on Tiger Moths trained by ex-RAF instructors who had drilled into me the danger of swinging any prop.
Last edited by paulross; 10th Sep 2018 at 14:27.