Sometimes it only takes a very minor shift in time and or space to dramatically change an image. This effect can be especially dramatic when shooting water or people. I saw this very clearly during a recent hike in the Sierra Nevada, when I shot two pairs of nearly identical pictures. For the first frame of each pair, I was standing up. For the second, I was simply squatting in exactly the same place. What a difference! Without keeping in mind how much things shift when you move even subtly in any direction, I would have missed the frames I like the best.