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.
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.