I will admit to having failed to put the mags off more than once. Our aircraft has twin toggle switches rather than a key, and the engine is first stopped by lean cut-off, then mags to "off".
What makes it worse is that i pulled it around by the live prop to get it back in it's farmstrip hangar.
The second time I made this mistake, as I leveled the wooden prop to avoid moisture balance issues.......the twin impulse mags clicked, the motor fired and ran in the hangar for 10-15s
Funnily enough I have remembered every time since. That sort of thing makes an impression, even on slow learners like me.
And look - I've still got all my fingers.
Fly safe !
HnH