Visible crew/equipment: When Hooper is underwater in the cage wearing scuba gear, very bright light reflections from the crew are visible on the regulator. (01:53:05)
Continuity mistake: When the shark bursts into Orca's cabin and tries to attack Brody, in the shots facing the shark, at the right of the screen is the forward window. The previously shattered glass (from the yellow barrel) is now perfectly intact within the window frame, and the large wood cabinet below that window is gone. The glass is shattered once again, when Brody climbs out of the cabin, through the window beside it. (01:57:55)
Revealing mistake: When Quint is gone, the shark bursts through the window of the cabin surprising Brody and in its second close-up, with its jaws wide open, the shark's head moves from left to right of the screen. As it does, the aft side of the cabin set is visible on the left, with the wall conspicuously missing and then at the fore side of the cabin, the window frame is not complete. (01:58:00)
Revealing mistake: When Hooper encounters Ben, the shape and size of the hole in the hull of Ben Gardner's boat, as well as the paint stripes on the hull, all change in this shot. (The close-ups of Ben's appearance were shot later in pick-ups after principal photography wrapped.) (00:49:30)
Continuity mistake: Throughout the film, Hooper wears on his left ring finger a gold bypass ring with a distinctive design and a small black stone, presumably black onyx. When Hooper is lowered in the shark cage, underwater the ring design differs; it's a classic style with a larger oval shape stone. (01:53:05)
Continuity mistake: After Hooper enters the shark cage, Brody takes Hooper's eyeglasses and places the left earpiece in his mouth, but when Hooper tells them to try and keep the shark off him till he's lowered, it's the right earpiece that's in Brody's mouth. (01:52:20)
Revealing mistake: As the shark jumps on to the back of the boat, a metal wire is visible from the shark's fin. It's not the barrels because it disappears in the next shot. (01:56:50)
Continuity mistake: At the harbor, just as Brody asks, "Help me get those guys out of the boat, will you?" in the wide shot there are five men standing on the dock and three standing in the boat. Then in the next close-up, there are four on the dock and four in the boat. The one in a black/white check shirt is now at the end of the dock and the one in a black rain slicker, that was standing on the dock, is now suddenly standing in the boat below, at its stern. (00:28:45)
Continuity mistake: Quint slams the machete into the wood beside the side cleat, and rigging lines. However, when the shark joins them aboard the boat and Quint is sliding towards its mouth, the machete's position changes - it is farther down towards the stern, note the black anti-slip mats. (01:44:30 - 01:57:00)
Continuity mistake: Before Alex's attack, the guy who plays with his dog Pippet on the beach wears a bright yellow Izod polo shirt with straight hemmed sleeves, in all of his close-ups. However, in all of the wideshots the polo he wears is pale yellow, with shorter sleeves that have the classic rib-knit sleeve openings. (00:14:10)
Plot hole: When Hooper and Chief Brody are trying to get the Mayor to re-close the beach after finding Ben Gardner's boat, they fail to mention they also found Ben Gardner's severed head. The Mayor would be forced to re-close the beach if yet another confirmed shark fatality had been mentioned, but Hooper and Brody never bring that important detail up. [This is still a mistake, but the explanation for this is that the scene where they find Ben Gardner's head was not in the original script. Originally, they just found his boat. Spielberg felt the scene needed a little more shock value so they shot the part with the head in a swimming pool long after the main filming had been completed.] (00:50:20)
Continuity mistake: When Quint goes below deck to get the machete, with the intent to cut the barrel lines, the machete he takes is shiny and clean, with no rust or pit marks at all. Yet, when Quint holds it over his head and then embeds it into the side of the boat, the entire blade is rusty and full of pit marks. (01:44:40)
Visible crew/equipment: When Quint hears the clicking fishing reel, he puts on the harness and slowly lifts the rod. At the start of the shot facing the reel, a crew member's hand and the top of his head are reflected in the circular chrome. (01:15:25)
Revealing mistake: At the boathouse, when Hooper cuts open the tiger shark, it is very noticeably a fake shark (note its sewn seams) in contrast to the real tiger shark hanging on the dock earlier. (According to "The Jaws Log," by Carl Gottlieb the local fishermen had been unable to catch a big enough shark for the dock scene, so they found a freshly caught 13-ft tiger shark in Florida then flew it to Massachusetts on a private plane, but by the time the scene was filmed the shark was already decomposing and had a foul stench). (00:43:40)
Revealing mistake: When the shark takes the fishermen's bait, the attached chain pulls one corner piling at the corner of the small dock. When the dock gives way in the wide shot, another line around the second corner piling visibly pulls it as well, in order for the dock to break apart effectively for the stunt. (00:26:25)
Continuity mistake: When Brody waits to board the ferry he says, "Charlie, take me over to those kids, will ya," and there's a painted board with 'Amity' attached to the ferry base. However, when they reach the other ferry landing the board with 'Amity' is gone, in sloshing water (even more visible in a deleted scene). It's the same side of the ferry, note the ferry Captain stationed at the permanent helm at only one side. The short haul Chappy Ferry's On Time scow was the stand-in for the fictitious Amity ferry. (00:11:40)
Revealing mistake: When Charlie and Denherder, the two fishermen, first wrap the link chain around the corner wood piling, the dock they stand on is clearly visible. A bit later when the shark takes their bait, the crossbeams at the base of the dock are actually gone for the stunt. (00:25:00 - 00:26:20)
Visible crew/equipment: After losing Quint, Brody is in the flooded cabin as the ship tips. Just before the shark bursts in, there is an exterior shot looking into the cabin through the window with the shards of glass. A crew member wearing a jacket and hat is visible in the reflection of the triangular piece of glass, at the top right of the screen. (01:57:55)
Continuity mistake: The damage to the bars of the cage and the top canister changes, and is quite different once Quint and Brody hoist it up. (01:55:05)
Factual error: After the shark tries to attack Brody on Orca, it swims backwards out of the cabin, just as it does in other shots. No shark can swim backwards. (01:58:05)
Suggested correction: First off, it wasn't a severed head as you can still see the body attached to it, and second, what difference would that make? Two people and a dog already died, with the death of the Kitner boy being witnessed by several people, and the beaches still stayed open because the mayor was too stupid to close them. Not only that, but during the scene you mentioned, I believe it was Hooper who said that THREE incidents had occurred BEFORE the third killing takes place on-screen.
And not only that, but the mayor witnessed Hooper saying that the shark they caught was not the same one, or at least it was possible that it wasn't. Either way, shark attacks were happening, but the mayor did nothing about it.
Brody states that "It was Ben Gardner's boat you should have seen it and it was all chewed up," yet failed to mention that they found his body on board. The whole scenario just doesn't make sense. How does Ben Gardner die? If I remember correctly from the book, his body is never found, and it was assumed that he had fallen overboard. Also, in an earlier scene, Ben Gardner is on his boat heading out from the dock that day with at least one mate on board; what happened to him/them?