Continuity mistake: When Chrissie goes swimming at the start of the film, we see the moon in the distance. However, when we see Chrissie from the shark's point of view under the water, the moon is directly above Chrissie.
Continuity mistake: In the famously later on added scene Ben Gardner's suddenly emerging head scares the hell out of Hooper the diver (as well as any viewer). What diminishes the effect is that the second close-up of the head looks nothing like the first, much creepier one.
Continuity mistake: When Quint comes out the cabin with the life jackets the boat is listing to our left as we look at it from the back. When he asks Hooper about the injection and the cage the boat has levelled off, and stays that way until they try and lift the cage back up.
Continuity mistake: Near the end, when Quint is struggling to get away from the shark, the nearby cans on the deck change between shots.
Visible crew/equipment: When the man is in front of Brody talking to him on the beach the film crew are reflected in Brody's glasses.
Factual error: Shooting an oxygen tank (or even an air tank) would not cause it to explode, rather the tank would just empty quickly, but it wouldn't blow up, no matter where the bullet hit.
Factual error: When Hooper examines the remains of the swimmer he lifts up what is left of the woman's left hand and forearm. The blood and flesh are a bright colour. Blood and flesh discolour very quickly on a dead body - these bits should be a lot darker and the flesh a lot paler and dark bluish in colour.
Continuity mistake: At the end when Brody is up in the crow's nest of the Orca waiting for Bruce to get close enough for him to shoot the tank in Bruce's mouth, notice that the crow's nest is sinking much faster than Bruce is moving towards him. At these rates, the boat would have fully sunken before the shark got close enough.
Continuity mistake: In the scene where Hooper visits Brody's home, he brings two bottles of wine. Brody pours himself a large measure, and then gives his wife and Hooper some. If you look at the bottle a) it seems rather full for the amount he's just poured, and b) the level changes from shot to shot. Sometimes it's above the label, sometimes below.
Continuity mistake: In the scene where Brody blows up the shark, off in the background there is land in some shots and then no land in others. At one point, there is land about half way across the horizon, then there is land across the whole horizon, and then no land, it varies shot to shot.
Continuity mistake: In a deleted scene, Quint walks into a music store to buy piano wire for his fishing reel. The woman behind the counter is giving a young customer a new clarinet reed and says, "It's a number three. It should be softer than the number two you had." Clarinet reed hardness is categorized with one being the softest and five being the hardest, so a three would be harder than a two.
Continuity mistake: When Quint is shouting 'Stand away from those damn cleets,' he is holding a hand gripper on the orca's mast. Then he's near the controls, which is impossible in a split second: no human can move that fast.
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.
Other mistake: When the boy gets attacked on the lilo and everyone is told to get out the water, the group of boys are standing just in front of the pool of blood. The water is too shallow for a shark of that size to swim in without it being seen.
Visible crew/equipment: When the first barrel goes on and Quint is at the front saying "5° port now." When we see a close up of the barrel at the bottom right of the picture you can see the wake from the camera boat. The wake is going toward the shark and not away from it.
Revealing mistake: When Hooper is lowered in the cage you can see the oxygen bubbles rising to the surface from the underwater camera team.
Continuity mistake: The word ORCA on the back of the boat continually changes from shot to shot varying from very rusty letters to shiny letters.
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.
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.