fire and brimstone
Been there, done that: bought ebay general replacement batteries for my Makita, set them in the Makita charger, and smoked up the garage. Plenty of fire to go with it, and lots of fire extinguisher residue to clean up.
Tesla cars have caught on fire.
I have 24 Nissan Leaf modules, they've proven very stable. They sit in the belly of the Leaf, NO air conditioning, no fan. I've not read of any incidents with them.
For my two Schwinn Tailwinds I've studied their Lithium Titanate literature and I like what they have to say, how the batteries perform, and how they're not the 18650 design.
But with ALL of them, my charging station sits away from the garage: it's a discarded cast aluminum bar-b-que that should contain the cannonfire when lithium goes incendiary. It's only been during a charging event that I've experienced the bad stuff.