In the scene that Jason shoots at the helicopter pilot , the window gets covered with the pilot's blood, when they show the window again it has no blood.
During the car chase scene, Fitch is seen driving a silver Toyota and a silver Hyundai throughout the chase, although they are supposed to be the same car.
During the car chase, Diaz's truck changes from a Suburban, to an Explorer, to an XL-7.
The piranha seem to vary in size throughout the movie to best fit the plot.
The three main characters are being held as prisoners at the Venezuelan (Spanish-speaking) military compound. They leave the building via a door marked in English, "ACLS Tool Room."
At minute 58:34 the Millenial Captain of the USS St. Harper orders a command: "Ready the depth charges" and then orders release the depth charges in the next scene. The US Navy hasn't used or carried 'depth charges' on DD's - Destroyers or DDG's - Guided Missile Destroyers since shortly after WWII.
At minute 34:00 and through more scenes going forward, the dead fish floating in the river are floating almost entirely above the surface, as would a Styrofoam prop, not like a fish.
A character is eaten by a buffalo-sized piranha that jumps out of, and then back into, water that is clearly only inches deep.
in the scene where the dive master is explaining the high powered gun, by looking closely, you can tell that it is only the nerf longshot painted in black.
The entire SEAL Team tasked with destroying the Piranha have long hair. Naval regulations make no exceptions for SEAL Team members to have long hair.
The iPhone does not have a battery that can be removed, especially as easily as it is shown in this movie.
At 1:23:00 Fitch throws Gordon out of the helicopter, with no SCUBA gear, the Fitch jumps out of the helicopter with no SCUBA gear. Next scene and both have magically found SCUBA gear to wear.
A man in a helicopter reacts to being shot at point-blank range almost a full second after the sound of the shot is heard.
In one part of the movie they are walking through the jungle and in the bottom of the screen there is a random water bottle.
As Jason Fitch is telephoning from Venezuela to Washington D.C., it is in Washington D.C. night - in Venezuela day. The two locations are in the same timezone.
During the underwater scene near the beginning that is supposedly taking place in the Amazon river, the diver (Fitch) is clearly swimming around a coral reef. There are no coral reefs in the Amazon.
Jason Fitch is taken from the Venezuelan airport to the Venezuelan military base in a Tahoe with California plates.
(At about 25:00) one of the scientists projects the piranhas expanding their range up various river systems from the Orinoco Basin "up into the Mississippi". There are no connecting river drainages anywhere in South or Central America where this is even possible.
The piranhas attack the Keys in the "Panhandle" of Florida. The Panhandle is the northwestern part of the state.
After the American warship bombards the piranha infested water, we see the characters test a lake. Whilst there are numerous dead piranhas floating in the lake, ducks can clearly be seen swimming. Ducks are impervious to warship missiles now?
Tiffany somehow makes the helicopter run on oxygen when it runs out of fuel.
Sarah says the fish are doubling in size every few days, while Brian counters her by saying it's "exponential" and they're doubling every 36 hours. Any factor that doubles itself at a regular interval is exponential.
The captain of the navy ship that is sunk by the piranha is about 20 years old and has long, greased-back hair. Another "soldier" has a thin, ratty beard.