Revealing mistake: In the scene where "More Than Words/Heaven" is sung, there is a montage of shots of Drew playing guitar on stage and Sherrie singing outside. One of the shots of Drew has been horizontally flipped - in it he's now playing guitar left-handed and his "The Bourbon Room" T-shirt has the logo back-to-front.
Revealing mistake: During the scenes where Tom Cruise and Julianne Hough are singing "Wanted Dead or Alive", several items in background have a mirrored image, including "The Bourbon Room" stage sign, an Exit sign and other signage. It does not appear to be an entire mirror image but rather just of a green screen.
Answer: The marks could be from when they were sliding across the pews.