Apple has today issued a recall for some wall adapters sold between 2003 and 2015. They were sold with both iOS and Mac devices. These recalled wall adapters are only the part that plugs into the wall, not the power brick itself. Apple says that the faulty wall adapters are only for models sold in Argentina, Australia, Brazil, Continental Europe, New Zealand, and South Korea. All others, including ones sold in the United States, Canada, and the United Kingdom, are perfectly fine.
According to the recall, in a few rare cases there was a risk of electrical shock if touched while plugged in. To find out if you have one of the recalled models, go to this page and see if yours is recalled or not. If it is, go to an Apple Store and they will give you a new model for free.