In the beginning of the movie, Cydney (Milla Jovovich) hangs her feet out of the jeep and they are almost hit by a tree while passing. A short while after, it is noticeable that she has red nail polish on her toenails. However, later on in the movie, in various scenes that show her feet, she no longer has nail polish.
In the scene where Nick and Cliff go 'hunting' it is pouring rain. When Cliff returns to the campsite he is completely dry and his hair is perfectly unscathed, however when Nick returns he is partially wet.
The headrests in Cliff and Cydney's Jeep are missing during interior shots then reappear during long exterior shots. Removing headrests from seats has been a common practice in movies for decades in order to provide more visibility while filming inside automobiles. This can usually be spotted by the two mounting holes at the top of the seat which are visible once the headrests have been removed.
SPOILER:A fter the hatchet falls down the cave it lands on the green boat. When Gina takes the picture and looks down in the cave the hatchet is no longer on the boat. But when nick gets back on the boat the hatchet is there again
Near the end of the movie, Gina receives a phone call from a supposed AT&T rep asking "about her long distance". 1) In the US, the cell phone carrier either provides or arranges the long distance. The customer has no choice. 2) Doing business in the US, AT&T is required by federal law to subscribe to a frequently updated list of numbers assigned to cell phones. The same law severely restricts who they can make telemarketing calls to. In short, AT&T's long distance division would never call her cell phone asking about long distance.
While in the sea cave, Nick says "What we have here is failure to communicate" and attributes the line to Paul Newman in Cool Hand Luke (1967). The line is most notably spoken by Strother Martin, but Newman's character DOES actually say this line in the film, mocking Martin's character.
A double rainbow is shown with red on the outside and violet on the inside of both rainbows. In reality, the order of colors in the outer rainbow is always reversed, with violet on the outside and red on the inside.
When Gina and Cydney are fighting on the rocks above the cave, they knock the hatchet down in the crevice where Cliff and Nick are. The hatchet flies down like someone aimed and threw it at Cliff instead of it just falling down there.