The scene where Sarah gets stuck in that horribly, horribly tight tunnel. Also, the shots of the girls (primarily Holly) crawling through it will make even those with mild claustrophobia feel very uncomfortable.
You don't even need to have claustrophobia. It's very easy to feel uncomfortable watching the girls squeeze their way through tight tunnels. Guaranteed to make you want to walk around and stretch your legs!
Sarah's hallucination of Juno in the car.
The scene where the crawler is right beside Sam and Becca as they're hiding, and it's making that horrible sound...
The deaths of Paul and Jessica. The noises as the pipes go through his head... and when the camera pans out, you can see that his hands are still twitching...
Does caving sound like a fun activity to you? You probably shouldn't see this movie then.
To play on this, an Austin area theater company held a special showing of the movie. In a cave.
The possibility that Sarah could have been hallucinating the monsters and it was her that murdered her friends.