I've posted this before in the main forums before, but because this is such an important safety issue, it deserves to be repeated when these topics appear.
I had been stealth vaping a mechanical BB mod at work. I would typically keep it in a pants pocket in my locker between vapes, between breaks. The BB has a protruding fire button, and when I put the mod into the pants pocket I was unaware that the fire button got compressed.
When I returned to my locker to have a vape, I felt the BB in the pocket and it was too hot to touch. I grabbed a wash cloth and pulled the mod from the pants pocket. The stench of vented gas was of melted plastic and electrical components. What I saw was melted plastic of the battery's body and both ends popped off.

Obviously, in hindsight what had happened was the battery became discharged too quickly because of the compressed power switch in the pants pocket. The battery went into thermal runaway.
There is no protective electronic cut off in a mechanical mod. The protection built into protected batteries might as well not even be built into them (IMHO). The only safety features the BB has are a hot spring and the fire button allowing gases to vent through it; the mod worked as it was designed to do. It was my error to not remove the battery between use, or detach the connection to the delivery device, or to not make the mistake of leaving the mod unattended where the fire button could be activated.
I would like to point out that the above battery was a so-called "protected" 14500. Since this incident, I no longer put my mods into a pants pocket unattended and practice more common sense with batteries. I no longer use protected batteries, preferring the safer chemistry IMR batteries.