Rigging a character is like being DEAF. If you get it wrong or have an issue, no-one knows about it, that is until someone starts screaming at you.
Like the example given here of Grout the golem, everything was going good until his head turned. That was when the animator noticed that part of the face was stuck in place, causing a bottleneck in the whole production process. Send it back to rigging right now!
There are many reasons why things like this happen. In this case a single pixel was not assigned to any one armature / bone. With nowhere to go, the pixel staid right at home, causing shot 53 of a rush job to stop cold.
Should this happen to you, here is a great place to utilize the BlenderPDX club. Got time, bring your file on a stick and come to the next meeting. The blenderheads will offer help like a hive of ants coming out for a drop of honey. In a rush, email us at Blenderpdx@gmail.com or on our Facebook page BlenderPDX for plenty of immediate contacts.