Apple is working iPhone users in Europe that some of the USB adapters could pose a safety risk. Apple has found that some of its USB adapters that shipped bundled with iPhones and were sold separately could overheat leading to a safety risk. The adapters shipped in Europe with the iPhone 3GS, iPhone 4, and iPhone 4S.
These adapters were sold from October 2009 until September 2012. The affected adapters have model A1300 printed on the on the bottom of the unit. You can see an example in the image above.
Apple will replace the potentially defective adapters free to affected users. Those near an Apple retail store can get replacements there. Apple is also offering refunds for users who paid for these adapters.