Errors in geography: According to the trailer, the action takes place in the Indonesian archipelago but the monkeys, parrots, parakeets and the anaconda itself portrayed in the same trailer are all native of South America.
Errors in geography: Kota Bharu is on Peninsular Malaysia and not on the island of Borneo and walking, rafting to Kota Bharu involves crossing the South China Sea which is impossible. What the writers may have been thinking of is Kota Kinabalu.
Factual errors: They are heading up the river but get into a waterfall.
Factual errors: Anacondas are not native to Borneo where the movie is said to take place. It is possible that the snakes were introduced, but that fact is never mentioned in the movie.
Factual errors: The boat captain explains that his tattoo is Korean for "A new beginning", however, the symbols are Kanji, which are either Chinese or Japanese.
Continuity: When Cole gets mouth-to-mouth, Sam's thong is showing. Then from a behind shot, you see no thong.
Factual errors: When Michel gets eaten by the snake the snake carries him up and swallows him whole without constricting. Anacondas usually constrict their pray even if it isn't putting up a struggle.
Continuity: After Ben gets eaten by the snake you see the group talking but none of them are wet, even when they just hustled out of the water to get away from the snake.
Continuity: When Ben is getting attacked by the snake he pops out of the water with no rips on his shirt. Then he goes under, pops up again and he is all ripped up.
Continuity: When Ben pops out of the water you see blood all over him. Then when the snake is throwing him in the air there is no blood anywhere on his body.
Factual errors: The 'stone spider' is actually a golden silk orb weaving spider, harmless to humans and not native to southeast Asia
Errors made by characters (possibly deliberate errors by the filmmakers): After the boat sinks, a member of the crew says "Hard Drive's ruined". The device being held in his hand, dripping with water, is in fact an ASUS 52x CD-ROM drive. Clearly not a hard drive.