Continuity mistake: Jack tosses the medallion to Will. After a shot of Will's hand catching it and Jack's reaction, in the front shot of Barbossa as he pulls out his gun, the leather strap is slung across his chest high up near his neck with the buckle at his lower left side. Yet in the close-up when he reveals the wound, the leather strap is lower on his chest and the buckle is now higher up at his right side. (02:00:50)
Continuity mistake: During the Black Pearl's ambush on Port Royal, they destroy a wall to the prison. Pirates in one cell manage to escape from a hole in the wall destroyed by one of the boat's cannons and Jack is stuck in his cell because the hole isn't big enough for him to fit through. As this happens, we can actually see the small hole when one of the pirates say "My sympathies. You've no manner of luck at all" as the other pirates escape the cell. Yet in the next shot as Jack is watching the other pirates escape, the small hole in Jack's cell is now smaller than it was in the previous shot.
Continuity mistake: After young Elizabeth tells young Will "I'm watching over you", there's a strand of hair over his ear, which appears and disappears randomly depending on the shot.
Continuity mistake: Young Will lays unconscious by the edge of a blanket. When the angle changes there's much more blanket behind.
Continuity mistake: Barbossa shoots Pintel and a hole appears in his shirt. This hole soon vanishes never to be seen again. (01:11:00)
Continuity mistake: Barbossa's scar changes appearance several times during the movie. At times it's almost completely gone, and sometimes it's even quite "thick".
Continuity mistake: In the overhead wide shot of the Pearl pursuing the Interceptor they are maybe three boat lengths apart. Before and after they are further apart. In general during that entire pursuit sequence their distances change too rapidly, from nearer to farther away (as seen from either ship towards the other), but that shot shows a particular discrepancy. (01:22:45)
Answer: The curse needs the blood of everybody who took a coin from the chest. All the other pirates have already contributed so, as the movie opens, the only blood needed is Will's, substituting for his father. During the finale of the movie, Jack takes a coin from the chest, adding himself to the curse, so his blood is now required as well as Will's.
Tailkinker ★
But I didn't see any blood on the coins, and none of the pirates cut themselves, even before Will became part of the mix.
Yes, the other pirates did cut themselves before Will came into it, off-screen. The lack of blood on the coins can simply be explained as most of it dripping to the bottom of the chest, it being washed away by storms blown into the cave, or by the fact that they didn't drop that much blood on it in the first place.
When they had Elizabeth they believed she was Bill Turner's daughter, but they all thought the curse had failed, none of them had cut themselves so it makes zero sense.
They had been collecting back the coins for years. During that time they repaid their own blood. All they needed was the last coin and the blood of Bill Turner to break the spell.
lionhead