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.
Revealing mistake: When the Voight character throws a bucket of monkey blood over his captives, you can see the blood makeup 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!
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.
Revealing mistake: When Gary is attacked by the snake, as it's wrapping around him, his arm passes right through it.
Answer: They didn't set it up. They were chasing after the anaconda anyway and were stranded. They still wanted to go after the anaconda after they were picked up, so they lied to the crew. They knew the crew wouldn't go after the anaconda on their own.