I use Lithium Polymer batteries in radio control planes, and the flammability of these batteries under high load (charging or discharging) has been known for ages. There have been several examples of R/C modellers losing rooms, workshops or garages to fire when charging LiPos unattended, and a lot of us use fireproof boxes (or old ammo cases) as a container when charging LiPos.
As an example, here is a video showing the results of overcharging a typical Lithium Polymer battery. This is one of many demonstration videos that did the rounds of the R/C forums soon after the introduction of LiPo batteries to the hobby.
The crux of the issue is that the more contained energy you require, the higher the likelihood of some sort of catastrophic failure. As energy demands from consumer devices has increased, batteries in turn have been asked to store more and more energy. This is all fine and fluffy bunnies if the battery manages to release that energy in a slow, controlled manner. But if you think about it a hand grenade is a contained energy storage device in its most basic form. It just doesn’t happen to have a method of releasing that energy slowly.
Batteries store energy people. Treat them with care.