Audio problem: Near the end of the film when the shark is in the control room he eats a man underwater. The man is heard to scream and cry out, and you can hear him taking breaths in between screams despite being underwater with no breathing equipment.
Revealing mistake: When the shark goes on a rampage at Sea World, whenever the fin goes above water, it is blatantly obvious that the fin is fake, and it is doubly obvious in some shots when we can see the welding line running right down the fin where the crew fused the two sides of the fin together.
Factual error: In the scene near the beginning of the film where a diver gets eaten by the shark, an unattached arm is left suspended in the water. Not one drop of blood was spilt into this water.
Revealing mistake: About two-thirds of the way through the film, there is a wide shot of Jaws swimming. Keep in mind that a shark swims by the moving of its body and tail. It's quite obvious to see that Jaws is a matted-in puppet as his tail moves only once and he is propelled a long distance. Also note the fact that the tail move is quite mechanical and not the easy motion a real shark would make, and his body doesn't move at all. The shark in Shark Attack 3: Megalodon looked better than this.
Revealing mistake: Matte lines are visible on the submarine as Mike and his girlfriend drive it through the reef.
Continuity mistake: When Mike is in the water doing the welding on the tunnel patch he is wearing a full face mask with the rebreather attached at middle, but when he and Kay are swimming away and go into the door back to the control room after being chased by Jaws, he is wearing just a regular snorkel mask with the rebreather in his mouth.
Continuity mistake: The small shark shrinks a lot when it's brought into the enclosure. If it were its original size, it would have stuck halfway out of the tank.
Factual error: Although continually referred to by the workers at the park as "he, " the captive killer whale kept there is obviously female. You can tell by the shape and size of the fin.