As a ham, I regularily use battery packs, usually 6xAA. One day I was skiing with an extra pack in my pocket, and felt things getting very warm down there. I had inadvertantly dropped the car keys into the same pocket!
By the time I had gotten stopped, unzipped 2 sets of zippers, pulled the whole mess out, and dropped into the snow, the damage was done. My car key was ruined, the battery pack was partially melted, and all 6 AAs were toast.
I had a brief flash of the worst that could have happen just before I got stopped. I could have been hurtling down the hill, out of control, trailing a large yellow and red flame with black toxic smoke issuing from my completely nylon ski suit. All the while being gleefully filmed by dozens of cell phone packing people on the lift hoping for a utube top ten video.
Totally my fault, lesson learned.. never put an unprotected battery pack where it might contact metal!