Probably when a battery exploded in a mod.
In 2012, most of the batteries available for vaping were protected ICR batteries. AW was selling the first unprotected high drain IMR batteries, but the vaper "experts" at the time were telling vapers not to use unprotected batteries. What they weren't telling us (or they didn't know) was the new class of batteries were "safer chemistry" batteries that were not as likely to flame or explode when misused or abused than the protected batteries.
Back to my story, I wore scrubs at work and had put my street clothes into my locker. My mechanical mod was in my pants pocket for safe keeping. What I didn't know was there were other items in my pocket which continuously pressed on the protruding fire button of the mod. The battery became over-discharged, heated up, and exploded. Nearly caused a fire in my locker but I noticed the smell and investigated. The mod was too hot to touch barehanded, so I got a wet towel and removed the tank from the mod to break the electrical circuit. The battery had blown both ends out, and the barrel of the battery had swollen and compressed like it had melted. The pants pocket appeared to have been singed by flames. Not a good pic below, but its the only evidence I have.
After that unfortunate experience, I did my own research on batteries and found that unprotected IMR batteries were not only a safer option but performed better, too. I was one of the leading advocates to sway vapers away from protected ICR batteries to unprotected IMR batteries. I wrote the original blog "Battery Basics for Mods: IMR Batteries or Protected ICR?" which several online websites published. Within a short two year period it became common wisdom to declare protected ICR batteries to be obsolete for the purpose of vaping.