Factual error: All propane tanks for the last 30 or so years have a safety valve which prevents them from simply releasing gas when the valve is turned on. Given these tanks looked brand new and Jake only turns them on, the gas would not have been released.
Continuity mistake: The cliffdivers' shorts change from blue to white.
Continuity mistake: When Novak hops in the boat to join the sheriff as she races off to rescue her children, he is wearing shoes/boots. Later, when they are pulling Jake and Kelly out of the water, Novak is barefoot.
Factual error: When Mr Goodman is telling Julie "no one release this piranha into the lake", he states that pygocentrus nattereri is "the original piranha, they used to swim the Colorado River right up until the Pleistocene epoch, I still don't know how you found one...alive!" The pygocentrus nattereri is a species of piranha native to South America, very common, not even on the endangered species list. (00:43:10)
Continuity mistake: The length of the rescue rope changes drastically.
Continuity mistake: The little girl steps on glass. Later in the movie when they are climbing across the rope, there's no cut on her foot.
Revealing mistake: When the swimmers are being attacked, some of them are not even panicking.