Towards the end of the film when Jennifer Lopez's character fires several rounds into the head of the snake, she is firing a bolt action rifle but not once does she cock the weapon, and she fails to pull the trigger for the last round.
When Serone uses dynamite to blow up a blockade across the river, a falling branch knocks the spare barrels of fuel overboard leaving them with only one for the remainder of their trip. However later the boat is seen from the back with all five barrels tied in their original position.
Sarone's rifle is a bolt action Winchester Model 70 Sporter, which requires manual cycling of the bolt to fire each successive round. In the film, the actors neglected to cycle the action, firing the gun as if it was a semi-automatic.
At the end of the film, when Terri shoots the anaconda, she is using a bolt action rifle. While many bolt action rifles can hold more than one cartridge, you must manually slide the bolt to eject the spent cartridge and load a new one. Terri never operates the bolt and yet she is able to shoot numerous times.
The snake very rarely dispatches his victims in a manner appropriate to a constricting snake such as the anaconda - a constrictor must first "latch onto" its victim, using its mouth. It would not just spiral around its prey. The latching on provides an "anchor" to facilitate the wrapping and consequent constriction.
Near the middle of the film, when the monkeys are "warning" the documentary makers, you can see in close-up shots that several of the monkeys are tethered to the branches they are perched on with a fine cord.
After the first anaconda breaks through the glass to get Terri, Serone shoots the snake in the mouth with a tranquilizer dart. As the snake reacts to the dart, you can clearly see the wires under him that are controlling his actions.
The goof items below may give away important plot points.
Crew or equipment visible
When Serone fishes for the anaconda and the struggle starts, he sends Gary to put the lights on the anaconda. Gary takes the right spotlight, but there is someone else operating the left light, yet all people are on the rear platform at that moment (Dr. Steven Cale is in his cabin, unconscious; Mateo is already dead).
At the end of the movie as a final scare we see eggs that are being hatched. Anacondas are viviparous - their eggs are retained inside their bodies, without shells, until the young are fully developed, when they emerge from the mother alive.
At one point, the snake wraps itself around Gary, causing him to spin. When the snake is about to wrap itself around his arms/shoulders, the guy throws his arm out and up, and it passes through the snake.