Apple have announced that there is a possible risk of electric shock when using a certain range of Apple wall plug adapters provided with various Mac and iOS devices. Whilst the incidence is low you should check your adapters as detailed in the link below.
Here is a link to a page on the Mac1 site that will assist you in checking if your Apple wall plug adapter may be affected by the warning. If you have one of these adapters please follow the link in the page for assistance in organising a replacement part for free.
If you prefer you can go direct to an Apple page:
I have already been through the process myself on the Apple page and it was easy and painless once I had my serial number.