Apple recently issued warning that their 5W European USB power adapter which was shipped with the iPhone 3GS, iPhone 4, and iPhone 4S back in October 2009-September 2012, should be replaced as soon as possible by the new charger as the early shipped power adapter “may overheat and pose a safety risk.”
According to the officials, the adapter which is causing problems is the A1300 model, which have the letters CE in solid gray. The new shipped adapter, which is obviously not affected, is an A1400 model and has the letters CE outlined in gray. You can see the pictures of both below.
If you have one of the devices (as described above) and currently live in the countries give below, then you should visit your local Apple store now.
Apple Inc states will replace the old adapter with new one by verifying your serial number, so don’t forget to take your phone along too. Want further information or assistance, feel free to contact Apple’s technical support.