Somehow, the puzzle author figured out a way to turn 12 people into 13. Count them.

This puzzle consists of 12 or 13 people - depending on how you look at it. As you can see, body tops are periodically moved to 12 stationary body bottom positions.

The animation has 2 positions - one with 12 people - the other with 13. If you divide the image into 13 columns, you'll see how the trick works - click here.