Revealing mistake: In the opening scene, the helmet closes on the guy's head. Inside there are spikes on all sides, but when his head falls to the ground the blood only comes out the bottom. The spikes should have also gone into the other side of his head and blood should have run down his face to the floor.
Revealing mistake: When Amanda is slowly getting up right after being thrown into the needle pit, if you watch very closely, in one or two quick shots you can see a small pad on the actress' back used to hold the needles in place. Slow motion helps, but is NOT required. (Seen in Uncut version, though it may be in the theatrical version as well.)
Revealing mistake: When the group finds the picture of Daniel and his father Eric in the house, look closely at the photo - it was obviously very poorly patched together with photoshop. The lighting on Daniel/Eric does not match the lighting in the background at all, and it's clear that they have been crudely "cut out" of another photo and inserted over the background image. The extremely poor, obviously fake quality of the photo is even jokingly mentioned in at least one of the DVD commentaries.