When they're watching the tapes backwards, they see Tristen Hiding the tapes under some rocks (where they found them). The tape they're watching is one of the tapes they found, but when she hid them, she was being recorded on that tape
When Jeff puts Erica in the closet, she is already in rigor mortis. If the timeline of the story is correct, Jeff kills Erica, puts her clothes on the floor, puts her in the closet, and 'reappears' to the group minutes later fully clothed. Rigor mortis would not set in that fast.
Jeff's van has colorful beads and what appears to be a rabbit's foot hanging on a noose from the rear view mirror as he is driving his group to the campsite. However, when Kim later takes his van to the store to get beer and later crashes the van on the way back to Jeff's house, the beads and rabbit's foot are nowhere to be seen.
When the tour group meets Kim in the cemetery, she is lying on a grave with the name "Treacle" written on the headstone, but when they move past the the headstone and see it from the other side, it clearly has the name "Further" written on it, and also looks much older. (This is due to an extra on the DVD version, which includes watching a section backwards to look for clues. These clues then point you to different parts of the film to solve a puzzle, the tombstone is part of that. There is also a shadow of a tombstone that appears as a cross and then just a normal tombstone, and also ghost images throughout the film.)
When Kim's looking for a telephone list inside the drawer, she finds the personal files for each person in the house. She throw away the list and holds the files with both hands. In the next shot, she is holding the list again in her right hand along with the files in the left hand.
When Jeff is digging the tapes out from under the rocks, he grabs them and looks at them, and one tape slips out of his hand and hangs over his thumb. In the next shot, they are all firmly in his hands.
The movie's action is set in Burkittsville, MD, which is in Frederick County, but at the end the characters are seen being hustled into what is clearly labeled the "Howard County Courthouse," which is located in Ellicott City, MD, nowhere near Burkittsville.
At the end of the film, when Tristen is hiding the tapes, she is being filmed. However, the camera moves as if someone is holding it. How can anyone hold it if she sent the rest of the group into the forest to kill the tourist team?
After Kim is gets her beer from the store refrigerator, a woman comes round the corner and bashes her trolley into her. However if you look in the glass of the door behind Kim as she gets her beer, you can see the reflection of the woman standing there and waiting for her cue to push her trolley round, into Kim.
The goof item below may give away important plot points.
At the end of the movie, Tristen wraps the rope around her neck several times, trapping her long hair between her neck and the rope. In the very next shot her hair is hanging loose on the outside of the rope.