Revealing mistake: When Jack rescues an unconscious Elizabeth, as he reaches to lift her, his wet white shirt is translucent. From just above the elbow we can see the pink of his skin showing through the sleeves, but above that it's very apparent that he is wearing another garment under the white shirt. Yet when he's on the dock, there is no other garment under his shirt, which is still translucent enough to see the two tattoos on each of his shoulders. (00:16:00)
Continuity mistake: While underwater, as Jack removes Elizabeth's water-logged dress, his left hand and arm are visibly clean, as well as when he has her on his shoulder at the dock. Then while kneeling over her on the dock, Jack's left hand and arm are extremely grimy and dirty. Yet when Norrington grabs Jack to bring him to Gillette, and while Jack holds Elizabeth hostage, his left hand and arm are clean once again. (00:16:10 - 00:18:40)
Revealing mistake: Jack did not rip Elizabeth's corset, as the sound effects suggest: the bustier's hooks were all already undone, and he just pulled the different pieces aside. (00:16:15)
Continuity mistake: When Elizabeth's maids lace up her corset, obviously both the back and front laces were pulled very tight, as this is the reason for her discomfort and eventual lack of air. After Jack rescues her, as she is on his shoulder we see the two sides on the back of her corset are far apart and the laces in between extremely loose. Yet, when she is lying on the dock, Jack slices the front laces on her supposedly constricting corset so she can breathe again... (00:16:25)
Continuity mistake: Murtogg and Mullroy rush over to help lift Elizabeth onto the pier. Murtogg drops his gun behind the two posts and Mullroy drops his gun in front of the two right corner posts. In the wide shot, Jack's holding the medallion and Norrington rushes in. Mullroy's gun isn't behind him, at the posts, and Murtogg's gun isn't positioned the same way he dropped it. When Norrington grabs Jack, Mullroy's gun is lying in front of the posts again. Also, the ropes' position changes in these shots too. (00:16:30)
Revealing mistake: In the scene where Elizabeth has fallen off the rampart into the water, Jack rescues her and Murtogg and Mullroy help lay her out on the dock. She is not wearing her breast 'enhancements', that she wears throughout the movie. Confirmed by Kiera Knightly on the DVD commentary. (00:16:30)
Continuity mistake: Jack cuts the ties of Elizabeth's corset and drops the knife down vertically, with the blade pointing up, right beside her head. In the wide shot, just as Norrington runs up, the knife is now horizontal. No one touched the knife. When he removes the corset, in Jack's shot, he hands it to Murtogg to hold and puts his hand down. In the next shot of Murtogg, Jack's hand is still on the corset. (00:16:35)
Continuity mistake: After Jack rescues Elizabeth, he slices the front ties of the corset she is wearing, and gives the corset to Murtogg. As he hands Murtogg the corset in the first shot, the corset is right side up, because we see the waist flaps at the bottom. In the next shot, as Jack's hand lets go of the corset Murtogg is seen holding the corset upside down with the shoulder straps at the bottom. In the next shot, a close-up of Jack, in the background Murtogg is holding the corset right side up, and the waist flap is seen. (00:16:35)
Visible crew/equipment: During many close-ups of Jack, Will and other characters, either two side by side reflectors or a single large reflector screen are clearly reflected in their eyes. (00:16:45)
Continuity mistake: After cutting the corset, Jack picks up the medallion in a close-up shot of Elizabeth and her hand is next to her head. In the next wide shot, as Norrington rushes over, her arm is stretched out, up past her head. In this wide shot, the neckline of her slip is different than the two close-ups, before and after. In her next close-up, her hand is back near her head again. (00:16:45)
Visible crew/equipment: After Jack rescues Elizabeth, Norrington shows up on the dock and points his sword directly at Jack. In the first shot facing Norrington, (as the sword points toward the camera, which is Jack's POV), the clear reflections of crew members are visible on the large underside of the crossguard, at the hilt of Norrington's sword, when in fact the only one who should be in the reflection is Jack, or possibly even Murtogg or Mullroy. (00:16:50)
Continuity mistake: On the dock, after the rescue, Jack sees the medallion on Elizabeth's neck. In the next shot as Norrington and his men come, we see that Jack's coat, hat, and other effects are lying scattered around on the ground. Yet later, when Mullroy bends down, he quickly picks up Jack's things to show to Norrington, and the gun, compass and sword are neatly placed on Jack's hat. (00:16:50)
Continuity mistake: Near the beginning of the film, after Jack rescues Elizabeth and her father gives her a jacket, she stuffs the medallion down her shirt, Jack then takes her hostage, where the Medallion is visible around her neck. (00:17:00 - 00:19:00)
Deliberate mistake: The East India Trading Company in reality, branded pirates with a 'P' on their foreheads, never on their arms, the way Captain Jack has it. It was deliberately decided to be historically inaccurate, feeling it looked better on Depp's arm. (Confirmed on commentary.) (00:17:20)
Continuity mistake: On the docks, when Norrington exposes Jack's scar and tattoo on his right arm, he says, "Well, well, Jack Sparrow, isn't it?" and in the wide shot Murtogg holds the middle part of his gun. In the close-up when Murtogg says, "He said he'd come to commandeer one," he holds it near the bayonet, yet in the next wide shot he holds it in the middle again. (00:17:40)
Continuity mistake: When Gillette puts the iron shackles on Jack, as Elizabeth offers her protest, in the shots facing them, there is a barrel with a green sack on top of it and two wicker rods against the stone wall. However, when Norrington says, "Indeed," then when Jack says, "Finally," and also when Jack says, "I saved your life. You save mine. We're square," the barrel, sack and rods are not there. (00:18:25)
Continuity mistake: Just before Captain Jack Sparrow threatens Elizabeth Swann, watch her medallion. In one shot the medallion is over her dress, but in the next one, it is under it again without anyone having touched it. In the following shot, when Jack puts the chain around her neck, it is out again. (00:18:30)
Revealing mistake: After Gillette locks the shackles on Jack's wrists, it is clear in Jack and Elizabeth's close-ups that the wider side of the shackles' distinctively shaped straight bar faces out at his wrists. However, when Jack is in the blacksmith shop, sitting at the anvil trying to break the links, the left shackle is actually around his wrist the opposite way. (This even occurs in other consecutive shots.) (00:19:00 - 00:21:30)
Continuity mistake: When Jack slides down the rope at the start, the arm that swung him round is facing Jacks left as he grabs the rope. Camera cuts and it is now instantly rotated 90 degrees. (00:19:00)
Continuity mistake: At Port Royal, Jack attempts to escape by holding Elizabeth hostage. After she puts his hat on in the first shot, while their bodies are extremely close, her right arm reaches under his left arm to go around his back in order to put the strap with the buckle on. Yet, in the next close-up, they haven't moved but now her right arm is over his left shoulder. Quite impossible between shots. (00:19:10)
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