Most of the incidences have happened while charging (over charging), a few have happened by a spare battery making contact with other items in a purse or pocket. Several have happened by a PV being activated in a pocket when not turned off. A couple have happened by the improper use of magnets as spacers, and most severe exploding when being activated. This would most likely happen the first time the PV is activated on fresh batteries or a new coil, atty, etc,etc (short). Most talked about was the Legacy and stacked wrong kind of batteries bought at Radio Shack. The shrapnel associated with exploding batteries is normally the end caps.
So while it has happened it is a long ways from being common, and most of the time human error was the root cause. While they can still vent, safe chemistry batteries almost eliminates the chances of a fire or explosion.
HOWEVER, IMO we will see the day when one of these sub ohm devices blows someones fool head off. Someone somewhere thinks their cheap unprotected Li Ion batteries can push 12+ amps just fine. (is probably even on this forum)