Revealing mistake: After Hooper delays Quint's shot of the very first harpoon into the shark, he finally shoots and hits the right side of the shark's dorsal fin. Problem is that in this particular shot there is another harpoon already on the left side of the shark's dorsal fin! This is the exact same shot seen later in the film, after Quint shouts, "Run him down Hooper!" (01:24:50 - 01:41:00)
Revealing mistake: When Hooper is in the cage the regulator hose is to Hooper's right (viewer's left when facing him). However, when the shark first appears and passes Hooper, in the shot facing him the hose is now to Hooper's left (viewer's right), and the word "repeater" on the poison shark dart is also backwards. This is a flipped shot. (01:53:30)
Revealing mistake: In the absolutely brilliant boat scene towards the end of the film, there is a medium shot of the rear of the boat when Jaws comes right out of the water. If you look carefully under the shark model as it leaves the water, you can see the mechanical wheel they used to maneuver the model.
Revealing mistake: The ropes from the two barrels attached to 'Bruce' the shark are tied to the stern cleats, but then Bruce pulls on the ropes and starts to drag the boat sideways and nearly submerges the stern of the boat. As the boat is being pulled sideways by the ropes, they are both shown to have quite a bit of looseness, or slack in them. How can the boat be pulled like that if the ropes are not taut? (01:44:05)
Revealing mistake: When Hooper is lowered in the cage you can see the oxygen bubbles rising to the surface from the underwater camera team.
Revealing mistake: When the Orca is sinking and the shark smashes through the glass to get Brody, the mechanical shark's eyes are too obviously fake-looking in these close-ups.
Revealing mistake: When Hooper has found Ben Gardener's head, he screams and drops the tooth and the light. Watch at the bottom of Ben's boat, a wire can be seen to anchor it down.
Revealing mistake: When Chief Brody starts shooting the shark, his cartridge is missing.
Revealing mistake: When Quint begins singing 'Ladies of Spain' while Bruce is chasing the Orca into the shallows, he turns slightly to his left and looks over his shoulder. When he does, you can see that the barrels are actually being towed by the Orca, due to the wake of the line doing the towing.
Revealing mistake: When the boy is attacked at the beach a man says "did you see that?" The woman behind him wearing the hat sits up with everyone else then looks directly at the camera and smiles instead of looking at the attack.
Revealing mistake: When Brody is stabbing Jaws at the end with the harpoon, you can see air bubbles under the rubber skin on top of Jaw's head - air bubbles or the blood bags.
Revealing mistake: Just after the first shark attack, Chief Brody and his deputy discover the beached remains of the nude swimmer. As Chief Brody hesitantly approaches the grisly scene, the camera cuts to a close-up of a mutilated female arm and other viscera, covered with a creeping cluster of small saltwater crabs. Inexplicably, a single crab falls out of thin air on the right side of the screen to join the others. A moment later, Chief Brody arrives at the scene, in the background. Obviously, a film crew member dumped a bucket of live crabs onto the body parts for this shot, but poor editing allowed one of the animals to be seen falling onto the pile.
Revealing mistake: When the shark's jaws come down on Quint's stomach, look carefully and you can see Robert Shaw using his hand to push the shark's nose onto his stomach.
Revealing mistake: Towards the end, before Quinn is eaten, the shark is "chasing" the boat with 2 barrels. You can see the wire from the boat dragging the barrels off to the right side of the screen.
Revealing mistake: As Quint smashes the radio, then when Brody yells at him, nearby land is reflected in the windows behind them. There is no land in sight both before and after this scene.
Revealing mistake: When Quint is getting eaten, the Orca is now a solid tilted movie set - look at the sides against the sea instead of Quint, and the Orca is not moving.
Revealing mistake: On the second day aboard the Orca, Quint fires a barrel into the shark and they start to pursue it. There are some overhead shots of Quint on the pulpit and during some of these shots, you can see some sort of tow cable. It's visible on the bottom of the screen between some of the supports of the pulpit.
Revealing mistake: When the shark swims away with three barrels, in the next shot all three men are standing at the back of the boat watching it. Look at the sea in front of them and you can see one of the barrels waiting to pop up.
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.