Revealing mistake: When the baby shark dies it is an obvious fake rubber shark with no movement before it died and no reaction from the watching crowd.
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.
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.
Revealing mistake: Very obvious fake dummy when Mike and his girlfriend view the dead body.
Revealing mistake: Matte lines are visible around Jaws when he crashes into the window of the control room.
Revealing mistake: When Shelby's corpse shows up in a Ben Gardner like jump scare, you can tell that the hair is not attached after they made a mold of Harry Grant's chest and face and edited it.