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.
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.
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.
Factual error: 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.
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: 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.
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. (01:08:16)