Proper delay management is crucial.
HD size video takes a good amount of processing power. Almost all devices in signal chain will introduce noticeable video delay. While picture is “held back” the audio is passed without delay resulting in a loss of sync. The effect is cumulative in subsequent passes. Before you know it even a casual observer will report that a sound of a closing door is heard before it is seen.
Most flat screen displays also introduce delay which makes them difficult to use in a professional environment. Proper sync is critical for editing, sound and multi-camera control rooms (trucks).
The issue can be mitigated to some extent by placing proper audio delays to “hold” the audio signal back in line with corresponding video.