Revealing mistake: When the teenage guy is getting attacked by the shark, the shark slams him up against the boat. He attempts to climb back on the boat and the shark pulls him down and the guy rips some wood from the edge of the boat. You can see the cut away part of wood that will be ripped off before he actually rips it off. (01:11:05)
Revealing mistake: There's one bit where we see the shark slamming into a boat and gliding along the side with its mouth open. Not only does the shark's latex mouth bend beneath the snout, narrowing from pressure, but within the mouth you can clearly see yellow hydraulic tanks to open the mouth. (01:22:05)
Revealing mistake: After Terry has been taken by the shark, her mum stops the speedboat, and starts calling for her. Then the shark starts swimming over to the boat. In a following side shot of the boat, just before the shark slams itself into the speedboats side, you can see a thick line attached to the front of the speedboat, which wasn't there previously, which is probably used to tow the boat to something off screen. (00:21:10)
Revealing mistake: In the shot of Eddie falling backwards out of Tina's Joy into the water, his hair is wet before he hits the water. (01:10:15)
Revealing mistake: When the divers are snorkeling underwater for lobsters and they find one and try to grab it the lobster they show is a southern lobster without any claws. Lobster from New England look very different from the one in the movie and have claws.
Revealing mistake: During the scene in the bar, the first shot shows some kid playing an arcade of some sort, and the shot continues to scroll from right to left. However, all the writing (ie. the arcade machine, t-shirts, chalkboard, and various signs) are all backwards, which means the shot is flipped.
Revealing mistake: In the overhead shot, when the shark is going under the "Tina's Joy" boat, you can see the shark is being driven by a propeller as the water behind it foams. (01:11:20)
Revealing mistake: When Chief Brody sees the driftwood in the water, he sits down and takes his shoes off to wade in. As he sits down, his butt is already wet, even though he sits in dry sand.
Revealing mistake: When the shark attacks the catamaran remains which only Jackie and Sean are on, as the shark hits, one of it's front top teeth is clearly bent in half. It remains back to intact in all other following shots. (01:44:30)
Revealing mistake: When the shark attacks the sailboats with only Jackie and Sean aboard at the end, you can see that it is a totally different mechanical shark right down to the bad paint job.
Answer: For what it's worth, in the novelization the shark is a female and pregnant with the offspring of the first movie's shark, but that's not brought up in the film at all.