Revealing mistake: Toward the end, when they have killed the shark and are all floating about in the water, you can see that the horizon is a painting. The water is even lapping up against it. (01:20:30)
Revealing mistake: When the shark is attacking someone under the water you can see a hole in the shark and divers fins showing that it's a puppet.
Revealing mistake: During the sequence when Michael has his underwater motorcraft and the shark attacks him, you can see in certain long shots the machine which works the shark protruding from under its tail.
Revealing mistake: In the scene where the shark has Michael cornered in the old, rusted boat; as he's about to use his oxygen tank to rocket up to the surface, the shark breaks through the wall and when it does, a large number of thick tubes can be seen running into the shark to make it work.
Revealing mistake: When the shark is rammed by the boat at the end, right before it explodes it is too obvious that the shark is a model. Some signs are: the oversized dorsal fins, its mouth is bent at an unrealistic angle, and it doesn't have eyes.
Revealing mistake: As the boat sinks you can see the rope attached to slow the boat sinking and control how it goes down. (00:10:45)
Revealing mistake: When the shark explodes, in the last shot (from behind the boat) which shows him exploding it is painfully obvious that the boat is only a model and not the real boat from previous shots.