'iPhone 5 Battery Replacement Program'

Accoding to Apple:

Apple has determined that a very small percentage of iPhone 5 devices may suddenly experience shorter battery life or need to be charged more frequently. The affected iPhone 5 devices were sold between September 2012 and January 2013 and fall within a limited serial number range.

If your iPhone 5 is experiencing these symptoms and meets the eligibility requirements [...], Apple will replace your iPhone 5 battery, free of charge.

If you're wondering what "may suddenly experience shorter battery life or need to be charged more frequently" entails, it's as drastic as having 60% charge and within a matter of minutes drop down to 20%. The same applies for charging the device. If you have an iPhone 5 that is experiencing this type of battery behaviour, head over to Apple's page and enter the serial number to check eligibility.