Continuity mistake: The dressing that is on the evil Dr Leticia Baines's forehead keeps changing. When she is first placed on Grogan's raft, the dressing is soaked with blood. In the next few scenes it just has a small patch of blood in the centre. After she swims to rescue the boy from his canoe, she gets on the raft and the dressing is dry. In the next two close-up shots the dressing is wet (with water) and then it is dry again.
Continuity mistake: In the scene where the kids at the Lost River Lake Resort are being attacked by piranha fish, Brogan's daughter Susie is hiding under a blue boat. She tries to lift the boat so she can use it to rescue her friends. It's too heavy for her, so she uses an inflatable boat instead. As she launches the inflatable off the jetty, you can see the blue boat in the water beside the jetty. Moments later, Brogan arrives and picks up the boat from where it was when she was hiding under it.
Continuity mistake: As Paul races the jeep down the highway behind the RV, he suddenly cuts out and into the oncoming path of a car, where he swerves to avoid a collision. But his jeep suddenly speeds up like a cartoon car, while the other vehicle drives goes opposite; obviously an editing mistake.
Revealing mistake: At the beginning when the teens break into the waste water plant and start swimming, the boy gets attacked then killed by the unseen piranha, and a large blood pool spills out into the water. The girl is but 3 feet away, but somehow doesn't' see the blood as she calls out his name and scolds him for not responding.