Whenever a Raspberry Pi is sent back to the distributor as a faulty unit, someone in the chain (we at the Foundation, the distributors, and now the factory in Wales) tests the unit to see what was wrong with it, so we can try to eliminate any problems in later production runs. We get very few returns, and I’m very happy to be able to say that nearly half of the returns we do get involve Raspberry Pis which don’t actually have any problems at all; in these cases what’s usually going on is that the user hasn’t flashed the SD card correctly (or, sometimes, at all!), or hasn’t connected it to a display device properly.
But occasionally it’s been challenging to find out what’s up with a broken unit. In these cases, we sometimes ask the customer to send us the power supply they’ve been using (and, from time to time, some other peripherals too) so we can dig a little deeper.
Everybody testing returned units used in the UK has noted a problem out there with some power supplies, all of which appear to be counterfeit Apple chargers. We’ve all compared notes, and we’re all seeing the same thing: some of you are using chargers you’ve bought in good faith which are not the real thing, and which don’t behave like a safety-tested, properly engineered piece of hardware.
We’ve found that there are three specific, different kinds of fake Apple charger popping up and causing trouble in the UK. We know that Trading Standards are already aware of these fake PSUs; apparently raids on suppliers have been carried out, but there are still plenty of them out there. Here’s some video which should help you check that yours is the real thing. If you find that your charger is a fake, take it back to the seller and raise a stink: and most importantly, stop using it immediately. You could damage your Pi or yourself (and anything else you plug into the PSU).
Updated to add: some very helpful links are appearing in the comments with technical details about these and similar PSUs, teardown pictures and video and so on. It’s worth swinging by to read them if you’re interested in learning more.