Today, Apple launched a worldwide replacement program for USB-C cables that were shipped with the 12 inch Retina Macbook, or purchased as stand-alone accessories before June 2015.
Cables with the issue can be identified by their labeling, which reads:
Designed by Apple in California. Assembled in China
Redesigned cables share the same text, but also feature a serial number.
Customers that supplied a valid mailing address at point of purchase will receive the cable automatically. Others can contact Apple Support, visit an Apple Certified Service Provider or any of the retail locations to get their cables replaced.