When the two boys go off to find a phone signal the snow on them disappears and reappears several times between shots.
When Allison hits Jill, causing a bloody lip, in the next shot there is no blood.
The film is supposedly set in Massachusetts, a state in the United States, but the thermometer outside of the washroom at the beginning of the movie is in Celsius only. The U.S. typically uses Fahrenheit for temperature.
When Alison first goes missing, the group are in fresh snow in the wilds. Only Jill and Phillip have left the vehicle, both in the same direction, yet nobody seems to think of following her footsteps.