Watching Inside Edition last night, I ran to my junk drawer.... to keep my 9 VOLT BATTERIES from touching each other. I'd never heard this before.... keep them loose in a drawer or a bag, and they can brush against each other and start a fire!
So, keep them in the package they came in, or cover the positive/negative posts with electrical tape.
It's apparently not an issue with all batteries, mainly 9 VOLT.
How did I get to be an adult, and have never heard of this?