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: 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.
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: 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 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.
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.