Here's the story. I had not had my mods very long. I developed a habit of storing my mods in my locker at work so that on breaks I could go to the locker room for some quick vapes. I tossed my BB into a pocket of my pants that were hanging on a hook in my locker. (I work in a hospital and wear scrubs while working.)
I return to my locker during a break and reach into the pocket to grab my BB. It was too hot to touch! I got something to remove it safely and could smell the odor of melted plastic. I cooled the BB under the sink and look at the battery.
The plastic covering of the battery is melted away and the ends are blown out. There was a black oily substance around the BB's switch, I assume that was a melted layer of paint from my black BB or whatever the battery vented out.
Anyway, this incident was totally user error on my part. I should have known not to place the BB with its protruding horn button in an enclosed space where it could accidentally trip the switch on. The BB did what it was designed to do by venting from the switch, and it continued to work for a few days after this incident. Then the switch stopped firing and needed replacement by Altsmoke, which they did free of charge, including shipping.
The BB, despite it smallish size, may not be a pocket-friendly APV. That's why I invested in the custom lanyard and now hang that from a hook in my locker.
By the way, the battery happened to be a so-called "protected" Trustfire battery.