Apple has determined that, in rare cases, the Apple 5W European USB power adapter may overheat and pose a safety risk. This adapter came with iPhone 3GS, iPhone 4 and iPhone 4S models shipped from October 2009 to September 2012 in the countries listed below. It was also sold as a standalone accessory.

Customer safety is always Apple’s top priority, and we have voluntarily decided to exchange every affected power adapter for a new, redesigned adapter, free of charge. We encourage customers to exchange any affected adapter for a new one as soon as possible using the exchange process below.
Continue reading: Don’t forget - iPhone USB Power Adapter Exchange Program.