I mean, it's really rare (unless you're a moron), but mobile phone batteries can sometimes explode and/or catch fire. And while you're probably OK laughing off the odds of it ever happening to you, take a look at this kid's bed for the outlying exception.
Ariel Tolfree, 13, says her Samsung Galaxy S4 slipped under her pillow while she was asleep the other night. And promptly caught fire, burning right through the sheets and melting the phone into a charred black husk.
If there's a warning to be had from all this, let it be this: the offending battery was a cheap replacement, not an official Samsung one. You know how in the back of manuals companies always say "only use official batteries", and you laugh? Yeah, most of the time that's just garbage, but sometimes, they have a point.