Revealing mistake: When the shark bursts through the window of the cabin, in each of the shots facing Brody, including when he shoves the air tank into the shark's mouth, the glistening water is visible in the window's reflection behind Brody. The shark's reflection is never seen behind Brody, despite the shark's massive size in the small cabin. Bruce must have had the day off during Roy Scheider's takes. (01:58:00)
Revealing mistake: While Quint is in the shark's mouth, the comfy white cushion is visible under his back, protruding from the shark's mouth. (01:57:30)
Revealing mistake: In the scene in which the two men are fishing for the shark with the roast and inner tube, watch the amount of chain that is pulled off of the pier. Not only do numerous feet of chain go out but there are already many feet in the water connected to the tube. When the pier gets pulled into the water and the shark makes the turn to chase after Charlie, watch how close the pier is to him. With the amount of chain that was let out, the shark would have been well past Charlie and the shark probably would've beached itself.
Revealing mistake: After the shark disappears, with Quint in its mouth, in the next shot facing Brody the door to the cabin closes. The wire attached to the door, leading offscreen to the right, is clearly visible. (01:57:40)
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: Just as Quint finishes scraping his nails on the chalkboard, the camera begins to move in closer when he takes the first bite of the cracker. On the right side of the screen under Harry's chin, Roy Scheider is ducking down while staring at the camera, as it pans toward Robert Shaw, and Roy then lifts his left hand up, cueing Robert's line. (00:20:55)
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)
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)
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)
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)
Revealing mistake: During Chrissie's attack, she clings to the buoy and cries, "Oh my God! Oh God!" In the next shot, as she is pulled away from the buoy, a wire (at about the height of the bell) that shimmers in the moonlight and bounces up and down with the buoy, runs from the buoy towards the right of the screen. It is not visible in any other shot. (00:04:35)
Revealing mistake: While Hooper is below in the cage, he sees the shark nearing and up on deck as the barrels speed closer, the black triangular device attached to their lines is clearly visible, right in front of the barrels. The device pulls the barrels, not the shark. (01:53:15)
Revealing mistake: Towards the end of the movie, when Quint is practically bitten in half, the mechanical shark's rubber teeth bend as it bites down, and they also bend when the shark bites down on the tank. (01:57:10 - 01:58:05)
Revealing mistake: When Brody tries to hail the Coast Guard, just as Quint smashes the radio, the reflection in the center window is of the three windows on the opposite side of the cabin. Though the two side windows being reflected are normal, the entire center window being reflected is blacked out, reason being that the camera filming this shot is positioned in front of that window and in order not to see the outline of the camera, the center window directly behind it was covered. In the previous and following shots obviously there is nothing blocking that window. Similarly, this occurs when Quint speaks with the Coast Guard. (01:38:25)
Revealing mistake: When the bottom half of Quint's body is in the shark's mouth, Quint's fake belly shifts around where the shirt is open and the edge is also noticeable under his shirt. (01:57:25)
Revealing mistake: When Quint has decided to lead the shark back toward shore, after Quint pushes Hooper away when he tells him to shut up, Hooper tells Brody to hold on. In the next shot showing the three men, with the barrels in pursuit behind them, Quint moves toward the left of the screen and the line pulling the three barrels is clearly seen in the boat's wake behind it. (01:48:00)
Revealing mistake: As the shark pushes his face into Hooper's cage, in a very nice close-up of the underside of his pectoral fin the large amount of peeled paint is quite visible, as it is in other shots of other areas of his body. Then in another shot at the same angle, just before Hooper escapes the cage, the paint job is perfect. In some other close-ups the shark's outer flesh creases deeply, because of its hollow interior, when pushed up against other surfaces. (01:55:00)
Revealing mistake: After Hooper's comment, "He's working his way up to us," Quint rushes to the stern and shoots another line into the shark. In a very tight close-up of the shark's gills it is very apparent that they are just wide carved ridges, with no slits whatsoever, to even attempt to give the illusion that they are real, like in other great shots of Bruce. (01:43:50)
Revealing mistake: While the shark's face is up close and personal in the cage with Hooper, the equipment under the shark is visible as the camera pans back and is visible again in following shots. In the shot just prior to Hooper swimming away the device is quite clear. (01:55:00)
Answer: Possibly a scar from having his appendix removed, I've been told.
The appendix shot is Brody - he is feeling inferior as the other two share tales of the sea and the only scar he has is from his appendix being removed.