So, coincidentally as we're on this topic...
I was driving last night and my thigh area started getting kinda warm, but I didn't think anything of it. Then slowly it got warmer and then all of a sudden I smelled a bit of burning, and something HOT burned me from inside my pocket. So I was like WTF?! I reached into my pocket and pulled out a spare 18650, and it was so hot I could barely even touch it with my fingers.
It wasn't in a mod or anything either, just a single 18650 battery I keep in my pocket as a spare in case my evic dies. I left the battery outside my front door and came back to it this morning, being curious as to why that happened. The battery still looks brand new. No cracks or leaks. I only got this about a couple weeks ago from ESV.
The only other things in that same pocket with the battery was a standard Bic lighter and some change. I wonder if it was due to friction or some sort of freak accident chemical reaction with the lighter and change?
Needless to say, I just threw out the battery and won't be keeping spares in a pocket near my crotch area anymore.