Seattle Times [H/T Rob Devlin]:

A passenger’s cellphone caught on fire inside the cabin of an Alaska Airlines flight from New Orleans to Seattle that had landed at Seattle-Tacoma International Airport Monday evening.

It was a Samsung Galaxy A21, according to Perry Cooper, a spokesman for the Port of Seattle.

And:

“After much digging, I can tell you that the phone was burned beyond recognition”

How does this sort of thing happen?

From this How-To Geek explainer:

Lithium-ion batteries contain a ton of Li-ion cells. Each of these cells has a critical temperature—think of it as a boiling point. When the critical temperature of a cell is reached (due to external heat, overcharging, damage, or poor manufacturing), it enters an exothermic breakdown. Basically, the cell itself starts to release a ton of heat.

One sign of a potential problem is a swollen battery. If you spot signs of a swollen battery, follow up. If it’s an old, Mac, for example, back it up, then take it to the Apple Store.