I woke up one morning to a hissing sound. The Wismec NC Luxotic Mod on my bedside table was autofiring. When I picked it up it was uncomfortably warm.
I pulled the batteries and took it outdoors. When I put the batteries back in it started auto-firing again - AND THE 10 SECOND CUT OFF DID NOT ACTIVATE - it continued firing until I pulled the batteries.
I stripped it to have a look (although I know nothing about circuitry) and could see nothing obvious like damaged/burnt components, but juice had got inside the mod and the circuit board was soaked in juice. I checked the batteries and they were fine, although they were in a suspiciously low state of charge (I think they were almost fully charged the night before but I couldn't be 100% sure - but they did feel hot when I pulled them from the mod.
I threw the mod, and after the auto firing experience I've got into the habit of turning mods off.
In over 5 years it has only happened to me once, but it was something that was bloody unsettling - personally I recommend you turn them off whenever you aren't using them or put one in your pocket.