Thankfully the "boom" failure mode is the most common these days.  Back in the late 90s, when I worked full time for a company called "Battery Works" (no longer in business) we did everything from tiny 3 gram silver oxide watch 
batteries up to multi-ton forklift and floor scrubber 
batteries.  Those were the days when lithium batteries had 2 modes of failure.  I called them "firecracker and road flare"  Road flare was more common.  Basically it was just elemental lithium reacting with atmospheric water in a very bright and hot manner.  Firecracker is what this thread is about.  But always remember:  It could be worse!
Personally whenever I get around to building/modifying things, I'm actually going to stick to Ni-CAD and Ni-MH cells.  They can put out nearly inifnite current when needed, and love to be drained fully.  Also cheaper.  I could probably vape for a month on a stack of 3 8500mAh SAFT D-Cells.  3.6v@8500mAh 
And don't get me wrong.  Li-Ion and Li-Poly are great systems.  I just prefer that they are designed and implemented in a controlled system by a team of paid engineers who know exactly how they're going to be used and charged.