Face Unlock which uses the front facing camera on a phone to recognize the owner's face and unlock the unit. However, the method had a major flaw which was that a picture of the owner put in front of the front-facing camera could fool the lock into believing that the owner was trying to unlock the phone and could allow an unauthorized person to have access to the device. That led to an additional step to prove that it really was the owner and not an image that was in front of the camera. Asking the owner to blink in front of the camera is one way to prove that a picture isn't being used. But a change in appearance, like the addition of a beard, can prevent the phone from unlocking.
Apple has filed a patent for its own version of Face Unlock which uses the mouth, the tip of a nose and eyes, and the distances between your facial features to determine if you are the owner of the phone.
source: PatentlyApple via MacRumors