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)
Continuity mistake: When the shark is boarding the boat, Quint and Brody are on their backs in the cabin beside the table. It is clearly visible that they are lying on the very solid floor with the hatches in perfect condition, despite the visibly massive damage to the floor and hatch when the engine room exploded. (01:57:00)
Visible crew/equipment: After Quint smashes up the radio, the shark re-appears. In the shot of Quint, as he walks along the side of the boat, the clear reflection of the filming crew's vessel is visible in the nearest window. (01:39:10)
Continuity mistake: Just as the first cleat flies off the stern of Orca, the shape of the small metal hood differs and the entire wood rail at the top of the transom in this close-up differs from the rail in all other shots. The long wood rail has another small piece of wood attached at the end, forming an 'L' shape with a 90° angle, whereas in all other shots the rail is one long solitary piece. (01:44:55)
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.
Visible crew/equipment: After Brody buys the sign supplies, when Hendricks shows up Brody places the box in the front seat, and as Hendricks tells Brody about the Boy Scouts, pay attention to the left window across the street, where we can see one of the local residents holding a camera and looking out the window facing the location shoot, taking pictures of the actors and film crew. The filming took place at the corner of Main St and S. Water St, Edgartown. (00:10:50)
Continuity mistake: When Brody sits on the beach watching the crowd in the water before Alex is killed by the shark, he wears a watch on his wrist which disappears a few shots later when he believes he saw something. (00:14:48)
Visible crew/equipment: After tying off the cleats, the shark pulls Orca causing it to tip. As Hooper and Brody fall backwards onto the deck, in this close-up the white tape mark is visible on the black anti-slip mat - where Quint will land the machete in a subsequent shot. (01:43:00)
Continuity mistake: When Hooper asks Quint about being on the Indianapolis, both of his shirt buttons are unfastened. When Quint answers & starts telling the Indianapolis story, Hooper's bottom button is fastened. (01:29:20)
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.
Audio problem: While Chrissie is being attacked at the start of the film, when her mouth is visible it is not in sync with her screams. (00:04:15)
Visible crew/equipment: Aboard the Orca, Brody accidentally causes the cage bars to fall over, and as both air tanks roll forward the multiple head set lights are reflected on the two tanks. (01:13:35)
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)
Continuity mistake: When Brody fires the last shot at the air tank, in the next close-up of the shark, the tank lies parallel to its body deep within its mouth, with only the base of the tank visible between its teeth. However, in the next wide shot, the tank lies perpendicularly in its mouth, with the pressure knobs at its right. (02:00:10)
Continuity mistake: During the shark's attack on Orca, the tall, large steam pipe clamped to the wall, beside the bridge deck ladder, actually differs from the steam pipe seen in previous shots. (01:49:55)
Visible crew/equipment: Hooper and Brody grab a couple of poles as the Orca pulls alongside the two floating barrels. When Quint says, "Watch those barrels, boys. Watch them," the crew's vessel is clearly reflected in the cabin's side glass window. (01:42:15)
Continuity mistake: When Hooper pulls the tooth from the hull of Ben Gardner's boat, he wears his watch and ring on his left wrist and hand. However, in the close-up as he drops the flashlight, knife and tooth, he wears them on his right side. Flipped shot. (00:49:35)
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)
Continuity mistake: After the shark leaves with Quint, Brody is in the flooded cabin as the boat starts to tip and the stove is behind him. The large metal side panels are fitted in the grooves between the lid and base. However, when the shark crashes through the window the panels are gone and the lid to the stove is closed. (01:57:45)
Continuity mistake: When Brody tries to hail the Coast Guard, Quint smashes the radio and the stove area beside them is visible. When the shark re-appears Quint heads for the stern, and the stove area, which is visible as he walks, is completely different. (01:38:25)
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.