Chartered Trading Standards Institute, a UK-based consumer safety company tested 400 chargers they purchased online and found that just 3 of them had adequate insulation. This issue is increasingly coming to light as we discover that many devices sold by Amazon and other online retailers are not all they're cracked up to be.

Wondering if you have a dangerous charger? Trading Standards, via the BBC, has some pointers:

How to spot a dangerous fake charger

Plug pins - Plug the charger into a socket, but don't switch it on or connect to a device. If the charger does not fit easily, the pins may be the wrong size. There should be at least 9.5mm (0.3in) between the edge of the pins and the edge of the charger
Markings - Look for a manufacturers' brand name or logo, model and batch number. Check for the "CE" safety mark, but be aware it can be easily forged
Warnings and instructions - User instructions should include conditions and limitations of use, how to operate the charger safely, basic electric safety guidance and details of safe disposal

The problem with all of this is that these shoddy manufacturers are shameless. They'll forge all the CE markings and overall do a good job of making their power adapters look like the real thing on the outside while on the inside they are dangerous junk. At this point, there is no way I'd buy a charger for any Apple product from anyone other than Apple. I fully expect in 2017 we'll start discovering how many online venders are also selling crappy USB-C adapters and cables as well. Be careful out there.