Revealing mistake: During the scene on the banana boat, the woman who is attacked by the shark quite deliberately shoves her leg into the fake shark's mouth as it missed her when it came up out of the water.
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: As the shark grabs the woman off the banana boat and slams her in to the water, the fake shark's rubber teeth bend. (01:05:20)
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: When the shark bites off Sean's arm, the outline of his arm hidden under the coat is visible. (00:08:30)
Revealing mistake: When Hoagie lands the plane on the water you can see they are near land when they in fact are supposed to by far out on the ocean. The sand bottom is visible. Also a sailboat suddenly appear, it wasn't there earlier. (01:11:55)
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: When Jake falls into the water by accident and lands in the sharks mouth, you can see a pole attached to the side of the shark used to turn the shark and the body side to side.
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.
Suggested correction: She doesn't put her foot in its mouth. She instinctively tries to kick it or she's just unlucky with where she moves her foot.
Mortug
She put her leg in the shark's mouth. It's too obvious even without slow motion.