Very interesting issue with that. I had a 5 minute countdown video that was playing before our service started. I realized it wasn't synced to the background, so i clicked the sync button. Guess what displayed on the back stage? The video, except that it was playing from the start.
So my main screen showed 2:35 on the video, and the screen was 4:24 and counting down.
To not freak out my band, I fliped the mirror main screen until the countdown was finished.