I've noticed this with both BD and DVD on a few occasions. When it happens the movie will play quite noticeably out of sync. Usually I've found if I eject the disc and then re-insert it, so it starts at the menu not just where left off, it will go back into sync. I'm not sure why this happens or how this corrects it, but unless it's just a bad mastering or disc it has worked for the majority of times I've seen this.