During the three-man sword fight on the island where the dead man's chest is found, on top of the church, the characters are all fighting on a little shed-like part of the church. Right after Jack tells the former commodore, why he is in the wrong, the former commodore says "Enough!" and takes a swing at him. Jack rolls, then does a flip off onto the ground landing on his butt, or back.

There are about three things wrong with this scene...

When you see Johnny Depp dip down right after the former commodore takes a swing, it switches to Johnny Depp's stunt double, and if you really take a good look, you can completely see it is not Depp.

Second, when the stunt double jumps off the roof toward the ground, he does one flip and right when his head is about eye level with the door, and you can see the stunt double is already facing more forwards, and not backwards, it cuts to Johnny Depp sitting up from the fall. There's no way he could decide to reverse his motion to fall on his back, and since he is already eye level with the door, there's no way he'd have time to make another safe flip, onto his back.

Third, when Jack looks back up at the former commodore and Will fighting, you can clearly see that there is virtually no place Jack could have landed without breaking his bones on the rubble below.