Blooper

In several of the "summer" camp scenes it is clearly early spring. Many of the trees still have no leaves.

The water skier is doing his runs on the river. He is wearing a orange life jacket, but when he falls into the water the jacket is yellow.

When Kevin McCarthy is pulled into the boat after being attacked, his legs show no sign of any damage.

As Dr. Hoak is attacked by a piranha, he gets onto a raft and his sweater alternates between torn and intact.

At the climax, when Maggie is counting to a hundred before pulling Paul out of the water, her countdown is way off. She's either way behind or far ahead of the allotted time.

While speeding to the camp in the stolen squad car, Grogan says that the water in the "pond" was salty.

A pond is a natural formation. A man-made, concrete formation is a "pool."

Jack (Keenan Wynn) could not possibly have died in such a gruesome fashion. Human instinct is to pull back from a sharp pain. It is automatic. He was firmly seated on the dock. To die with both legs gnawed to the bones like that, he would have had to pull his legs back, and then, instead of crawling back from the edge (again, human instinct), dipped his legs BACK in the water. It simply isn't feasible. Self-preservation instinct would override it.

Paul holds his breath for almost four minutes with extensive physical effort under twelve feet of water. While the average person holds it about thirty to ninety seconds, Paul is physical fit and man can hold his breath three to five minutes.

Despite their razor sharp teeth and fearsome looks, real piranhas do not eat humans. However, it's explained these are genetically engineered piranhas meant to be introduced into the ecosystem of enemy territory. They are also said to be highly intelligent suggesting that the fish are getting revenge on humanity.

The speed boat is racing in the lake at full speed. The shot cuts to the skiers POV looking at the boat and the pull rope is slack and there isn't any tension on the line at all.

The exploding boat blows up just before the speed boat impacts.

When Dr. Robert Hoak drives away in the Jeep, the tires make squealing sounds, as if it was driving on asphalt, when in fact it was a gravel road.

Looking at the Commanding Officers ribbons, they are up side down. However, they are turned over in his next close up.

When Grogan phones Dumont to warn him about the piranhas, Dumont is sound asleep in bed, even though the kids and other two supervisors are still awake sitting around a camp fire. Dumont is the lead supervisor at the camp, and he would not go to bed while everyone else was still up and about.

No one would SCUBA dive in open water, especially where there is so much powerboat traffic, without flying the red and white striped 'diver' warning flags on their boat and/or on buoys around the dive area.