What I've heard is that one cell can easily discharge more than the others and go under nominal cell voltage, and they'll get a reverse polarity charge from the other, not-as-discharged cells in the pack. That seems to be a prime suspect in explosive discharge of some chemistries of LiIon (there are several). And the gases are -very- toxic (hydrofluoric acid?).

Some flashlight guys now insist on single cell LiIon lights, usually using 18650 cells.

Also, regular fire extinguishers won't put these out.

If you look at the TSA site, I believe passengers are limited to how much LiIon batteries they can carry on, by weight. I think this accommodates normal phone and laptop use. I suppose they figure at least they'll know if a battery fails in flight, although it would appear your problems aren't over.

People like pro videographers are forced to juggle their large LiIon packs between carry-on and luggage, I think.

These batteries are jugs of highly reactive chemicals. I'd hate to see a palletload in a plane get knocked around by an airpocket.