Ken Shirriff @ righto.com has done a detailed teardown of a counterfeit macbook charger and show us the difference from a genuine one and why this can be dangerous. He writes:
What’s inside a counterfeit Macbook charger? After my Macbook charger teardown, a reader sent me a charger he suspected was counterfeit. From the outside, this charger is almost a perfect match for an Apple charger, but disassembling the charger shows that it is very different on the inside. It has a much simpler design that lacks quality features of the genuine charger, and has major safety defects.
Counterfeit Macbook charger teardown – [Link]