Revealing mistake: In one shot when the boat is pulling away from the waterfall, you can clearly see that the shot is the same one they used earlier in the movie. The water from the waterfall now is going up the cliff! They have just put it in reverse.
Factual error: During the FIRST battle with an Anaconda, the 55-gallon fuel drums lashed to the deck of the riverboat are swept over the side, where they sink to the bottom of the river. This plot device drives the rest of the movie, since the boat must make a dangerous detour through "Anaconda country." There is just one small problem with this: Fuel drums full of diesel fuel OR gasoline FLOAT on water! The fuel in them makes them buoyant. It would have been a simple matter for someone to get a rope or a boathook and retrieve the fuel drums before they floated away.
Visible crew/equipment: When Jennifer Lopez is in the cabin with John Voight and the snake breaks through the window, if you look closely, you can see the wire that is keeping the face up.
Factual error: All throughout the movie, the snake makes a screaming sound. Snakes don't make sounds like that.
Revealing mistake: When the Voight character throws a bucket of monkey blood over his captives, you can see the blood make-up on them before the 'blood' is supposed to hit them. In fact, the 'blood' doesn't go near them - it is thrown diagonally so that it does not hit the supposed target!
Continuity mistake: After eating Paul Sarone, the anaconda has absolutely no lump in its body until it goes to spit him out, and even when it appears, it isn't big enough to be a man. The lump would have been there to begin with, not just magically growing out of nothing.
Continuity mistake: Near the end of the film Eric Stoltz stops Jon Voight from shooting Jennifer Lopez by stabbing him with a tranquiliser dart. Jon Voight tries to strangle him but the tranquiliser kicks in and stops him. They put him to bed and his throat wound has started bleeding heavily. When they get back to him after killing the last snake, there is no sign of blood anywhere on him.
Continuity mistake: When Matao, the captain of the boat, is killed by the snake, his hair is down when the snake wraps around him. When it starts to eat him, his hair is in a ponytail.
Factual error: 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.
Factual error: At the end of the film, when the lady 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. The girl never operates the bolt and yet she is able to shoot numerous times.
Continuity mistake: In the scene where three of the men jump into dirty water, when the old bald guy gets out of the water, his shoes are clean and white, and his shirt is dry.
Continuity mistake: When J-Lo comes out to see the fireflies she is not wearing a bra, but when they change shots outside you can see a bra strap on her right shoulder and it comes and goes in continuing shots.
Audio problem: In the scene where Ice Cube was hitting the snake with a Maddox, you hear a metal on metal sound.
Continuity mistake: When Ice Cube and J-Lo are fighting the final battle on the boardwalk, Ice Cube hits the snake in the head with an ax a number of times. When the shot shifts to the snake falling, the ax is embedded in its head, but when the shot shifts back to Ice Cube he is still holding the ax.
Revealing mistake: When Eric Stoltz and J-Lo are watching the fireflies, the camera angle changes and the fireflies (and indeed all the animals you can hear) stop their activity, you can see if you look closely a net of Christmas lights in front of the camera which were used to create the flashing fireflies.
Continuity mistake: Right after Westridge is caught by the Anaconda and the tree comes crashing down, the boat is first right below it, next shot it is nowhere to be seen, even at a broad angle, and then back to being there again.