(at around 59 mins) When Josh goes missing and the two others are calling his name, Mike is wearing a plaid shirt with a white t-shirt underneath. When they change views his t-shirt is black. When they cut back it's white again.
(at around 1 min) When Heather is showing off the contents of her pack there is a survival book on how to stay alive in the woods. It could have helped them, but they never used it. However, the reason they can't escape the woods is a supernatural one. The Blair Witch has control of the filmmakers and the woods, and a survival book would not have helped there.
While it may seem that the batteries for the video cameras should have died without any means to recharge them, eighty minutes of footage between two cameras would not have exhausted the batteries, especially if they had spare batteries (implied during the drive near the beginning).
The three are lost in the woods but in one scene, about 25 feet behind them, a field can be seen through a small gap in the trees. The road is also visible as they try to find the trail.
(at around 59 mins) When they lost Josh, Heather's hands and fingernails can be seen. Even though the group has been in the woods for several days Heather's fingernails are perfectly clean.
It is early morning daylight when the group starts walking again, and it is still daylight when Josh berates Heather, "We walked for fifteen hours today, and ended up in the same place!" If this was true, it should already be dark. In Maryland, there are only about eleven hours of daylight in October.
Several times throughout, a character's voice sounds like it's right next to the camera when they're not the one holding it, sometimes even when they're far away.
(at around 2 mins) The film supposedly takes place in 1994, but in the supermarket, Michael displays an oatmeal-raisin PowerBar. These were not available in 1994.
(at around 1 min) At the beginning of the movie, when Josh is getting out of the car, a 1997-2003 Ford F-Series pickup can be seen in the background.
When Josh and Mike first confront Heather about getting them lost, Heather's mouth does not move when she replies.
In the house a red light flash (from a camera) can be seen through an open window.
After waking up to find three stone piles surrounding their tents, the group decides to walk south for the day. However, they continue to walk toward the sunrise, which would be east.
(at around 1h) In Mike's reference to "The Wizard of Oz" in which he asks "Wicked Witch of the West, Wicked Witch of the East, which one was bad?" And Heather answers "Wicked Witch of the West was the bad one." Actually both Witches were bad. The Witch of the North was the good one.
Instead of crossing the same stream several times, all they had to do was follow it until they found the edge of the woods, a basic outdoor survival skill that occurs to none of them. Apparently the characters grew up in suburban comfort and have little outdoors experience.
Surely Heather's camera had a playback feature. They could have found their way back by viewing all the footage she had shot up till they started having problems and just worked backwards.