I had an 18650 flashlight and saw a video of them spontaneously turning into a flare with rocket thruster like flames and got rid of it immediately. These batteries are scary powerful when it gives off the magic smoke.
That they are, energy is energy. I was part of a Battlebots team and that is where I learned the smell of various rechargeable battery chemistries when they burned :-). We also had an exothermic adventure with a battery pack we built, fortunately it was not at an event, it was in earlier testing, but it forever gave me a healthy respect for those batteries.