Recently, my grandson brought me a device, a USB power charger. He said it smelled like it was burning. So it did. To my nose it smelled of charred fiberglass PCB. It was one of those little cube sorts that plug into a wall socket and has a USB port. It was for recharging battery operated devices like cell phones, cameras, etc. It was working fine when he unplugged it but it was stinking up the room and had him worried about a fire.
When I looked it over, I noticed a spot on the case looked melted. I didn't see any screws or snaps. Obviously it wasn't meant to be opened again. No hope for it, I had to break the case to see inside. A little judicious application of screwdriver (-) and hammer to the seam of the two parts cracked it open nicely.
Inside I saw the likely problem, a diode had been over heated. This sort of diode is rated for 1 amp but that doesn't mean it won't get hot with that current. It will. Rather toasty, in fact.The PCB material has charred, hence the smell.
The plastic case nearest had melted too. 
The diode had gotten so hot it had melted the solder on the PCB.
It seemed to me like the charger was meeting it's specifications, but just barely. The charger was being used to recharge a portable battery bank, the sort for recharging a cell phone when it's gone flat and there's no AC to be found. These are typically about 7 to 20(!?) amp-hours and will take all the juice a USB-2 port can give, 1 or 2 amps (I know USB-3 can do more, but who has that?) So most likely this poor tiny charger had did all it could and still survived. Barely.
Would it have caught fire? Maybe. This was on the edge, but as far as I could tell the charred PCB area wasn't close to the high voltage bits. The take-away, just because it's rated for 1 amp doesn't mean it can do it constantly. These are not rated for continuous duty. But what ones are? It's a void in the specs.
