Continuity mistake: When Tia Dalma asks, "Would you sail to the ends of the earth to fetch back witty Jack and his precious Pearl?" her lips are only partially covered in black. However, when she says, "Then you will need a captain who knows those waters," her lips are covered entirely in black.
Continuity mistake: After the ship with the "bride spirit" is attacked by the Kraken, Will ends up underwater, and when he surfaces his hair is soaked and slicked back, as he climbs on the ship's debris. However, in the next shot, his hair is parted, stringy and falling to the front. (01:33:50)
Continuity mistake: After arresting Will and Elizabeth, when Cutler Beckett holds up Norrington's arrest note, the way the paper is folded and how he holds it differs between shots. (00:03:20)
Continuity mistake: While Elizabeth listens as Captain Bellamy tells his crew that the stowaway is likely naked, she drops the paint brush into the pail. When it cuts to the close-up, the paint brush handle rests much higher in the pail. (00:46:05)
Continuity mistake: At Tia Dalma's, when Will tosses the knife at the tabletop, there is a tall slim candle beside his right hand. However, in the next shot when Tia Dalma says, "Against the cold and the sorrow," the candle beside his right hand is now very short, though no time has passed. (This doesn't refer to the short, wide candle in both shots.)
Continuity mistake: When Norrington runs with the chest, he falls and his sword ends up beside shell-head when he also falls. After the quick shot of his headless body running with the chest, in the next close-up the sword is farther away from shell-head, just before his crab-like legs appear. (01:54:40)
Continuity mistake: After Bootstrap gives Jack the black mark, when Gibbs shouts, "Back to your stations, the lot o'ya," the cloth round his neck changes position, depending on camera angle. (00:17:00)
Continuity mistake: The actual position of the burning word "Tortuga" differs between the first shot as it begins to burn on deck and from the close-up of the flames. (01:04:15)
Continuity mistake: When Jack beheads the pirate, the chest lands between certain plants, but when he kneels beside the chest to unlock it, the plants differ - despite the angle of the camera. (01:50:30)
Continuity mistake: Aboard the Pearl, when Elizabeth reaches for the gun at the top of the stairs, her hand is clean as Jack appears. As she hugs Jack's leg, her hand/fingernails are black and filthy. (02:04:10)
Continuity mistake: At the edge of the cliff, in the first overhead shot, when the four men lift the coffin its "head" side is nearest the water. However, in the next shot the "foot" side is nearest, before they throw it in the water. (00:05:30)
Continuity mistake: When Lord Beckett is trying to convince Will Turner to go after Jack Sparrow, he picks up a red hot poker from the fire and says that they've marked each other. The shot cuts to Will who asks simply "How did he mark you?" at which point the shot cuts back to Beckett and the formerly red-hot poker is already now a much cooler black. Glowing hot iron does not cool that quickly in air. (00:09:30)
Continuity mistake: When Elizabeth sits in the rain, holding her bouquet of wedding flowers, many long hair strands hang down her forehead. However, when she runs to Will there are far less loose strands of hair, and the hair pulled back is neater. (00:01:25)
Continuity mistake: When the cannibal women place the necklace of many fingers and toes around Jack's neck, the digits of multi-skin color and size are tied and hang in a specific way. When Jack seasons himself with paprika, the digits are different. (00:32:05)
Continuity mistake: When Jack spills oil on his cuffed hand, his fingers and nails are filthy as he slips the shackle off. However, next shot, when his hand is free, his nails are not nearly as black. (02:13:40)
Continuity mistake: Tia Dalma gives Jack a jar of dirt (brown, clotty-ish), but later in the movie, it is sand. (00:53:10 - 01:57:30)
Continuity mistake: While Elizabeth hides, Beckett walks into his office and the wood chest is in the foreground; under the lid the paper sticks out, with an edge that is creased and has writing. When Beckett lifts the lid the crease is gone and the writing differs. (00:26:50)
Continuity mistake: In the scene where Jack meets Bootstrap, Bootstrap's hands are filled with barnacles but when he touches Jack's hand to give him "the black spot" his hand is suddenly blue with no barnacles. (00:13:50 - 00:15:30)
Continuity mistake: When Jack, Will and Norrington fight for the key, Elizabeth tries to stop them by "fainting". After she gets up after her plan has failed, Pintel and Ragetti are seen running past Elizabeth in the background with the chest. In the next shot, they run past Elizabeth again. (01:44:30)
Continuity mistake: When captain Jack Sparrow gets slimed by the Kraken, Jack's hat lands on deck, slime-free. In the next shot, the hat is in a glob of slime. (02:09:30)
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.