Lip flap
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"Lip flap" is jargon used to refer to the incorrect synchronization of a soundtrack (usually spoken dialogue) with a corresponding movie as a result of either unintentional technical inaccuracy or from poor dubbing techniques. The result typically leaves a filmed character moving his mouth when there is no spoken dialogue to accompany it as it has been removed or changed somehow in post-production, hence the term "lip flap" or "lip-sync error".
In broadcast television this difference in synchronization between audio and video often starts within the CCD cameras used to capture the video due to the video delay allowing the audio to get ahead. Throughout the broadcast delivery system every digital video effect (DVE)applied has the potential to delay video so broadcasters attempt to measuer that delay and apply the same audio delay to keep the signals in sync. Unfortunately there is no "watermark" in the audio and video signals to define when they are in sync so this is an open loop problem and any oversight in lip-sync correction (adding audio delay to match video delay)at any point from origin to destination will be cumulative. In broadcast television it is not unusual for lip flap or lip-sync error to vary by over 100 ms (several video frames) from time to time. New display technologies like Plasma, DLP, and LCD also delay video causing even more advanced audio in home theater systems where lip flap may exceed 200 ms at times.
