Apple has determined that, in very rare cases, the Apple three-prong AC wall plug adapters designed primarily for use in the United Kingdom, Singapore, and Hong Kong may break and create a risk of electrical shock if exposed metal parts are touched. This wall plug adapter shipped from 2003 to 2010 with Mac and certain iOS devices, and was also included in the Apple World Travel Adapter Kit.
As reported by Apple on their Exchange and Repair Extension Programs page.
Apple states that customer safety is their top priority. Affected adapters will be replaced free of charge.
To identify whether your adapter could be affected by this issue, compare it to the pictures on Apple’s page documenting this issue.