While Rob is proposing to her, Anna is trying to remove Sander's ring from her finger. In one close-up shot we see her hands doing this in front of her. In the next close-up shot, her hands are busy behind her back. It would be unlikely anyway that she would do this in front of her, as Rob would notice it. But it would be even more unlikely she would move her hands from front to back, as he would notice that even more.
When Sander tells Anna about his romance in Venice, every shot showing the talking Sander from the back is out of sync.