Deliberate mistake: When Will is bound and brought before "Chief" Jack, Will says, "The compass!" and there is a close-up of the compass, as it hangs to the right of the long striped cloth. However, in all other shots the compass hangs to the left of the red and white striped cloth. This is a flipped shot - note the large buckles. (00:24:00)
Deliberate mistake: When Elizabeth chains Jack to the Black Pearl at the end, it is with a shackle that uses a bolt and nut to lock, which would have taken a considerably longer time for her to place onto him. As we don't actually see her do it, we are to assume that she placed it on him as one would place a handcuff, quick and easy. (02:07:00)
Deliberate mistake: When Shishka-Jack falls, the rope is tied round his left ankle. When the pole gets stuck, in the second close-up of Jack spinning free the rope is actually tied round his right ankle (his spinning loose of the rope is also the opposite way). (00:42:00)
Answer: The jar was to put Davy Jones heart in. He can't touch dirt ever again since he failed to keep up his duties which means he wouldn't be able to get his heart back.