The fight scene on the uncharted island between Will Turner and Cutler Beckett takes the pair into the abandoned mill, and out onto the top of the water wheel, which then falls and starts rolling down the hill.

At this point, Jack Sparrow jumps into the inside of the wheel. When they get to a dip in the hill, Jack Sparrows feet end up through the bottom of the wheel. He sits up when they pause, and hits his head on a bar (supposedly holding the two sides of the wheel together). Later on during this scene, Jack is running in the opposite direction that they are going in the inside of the wheel - with no sign of the bar (which should be pounding into his stomach).