Doing some keying in After Effects CS6 (admittedly not my speciality) and noticing some glitches when using Keylight.
The problems seem to result from using "unpremultiply results". The problem is that keeping this box checked yields (generally) better results, until of course you get to the glitched frame. The glitch can be anything from a part of the keyed backdrop showing through, to random cropping.
In order to avoid it I'm finding myself having to un-tick the box, however the resulting key is considerably worse than with the box ticked.
Here's some example images (never mind the blurred face... did that at request of the client)