In the beginning, when Trevor Moorehouse, first takes out his chainsaw it appears small. When he is chasing the man through the woods, its much bigger.
The amount of lines written in Jules's e-mail changes.
When Julie is writing her father the e-mail, she is saying what she wants to write in her mind. At the end she says to herself, "Love, Jules," but types: "Love Julie".
When Julie falls from the wall with the rope, she lands on her back without her knees touching the ground. At the scene thereafter, when her friends are coming for her, she is complaining about her scrubbed knee.
When Julie sends her father the first e-mail, she is saying what she wants to write in her mind. At the end she says to herself, "Love, Julie," but types: "Love Jules".
When two are locked in the freezer, no breath is visible when they speak.
Near the end, when Drew swings the oar at the killer, the oar obviously does not actually hit the killer, but the killer reacts and falls as if he had been hit.
Possible SPOILER - When Jess runs out of the office after the cupboard door has swung open to reveal Doug's body, the actor playing the 'dead' Doug is breathing.
In the opening scene, when Trevor Moorehouse starts the chainsaw, it is obviously a fake one. Only the sound is heard but the blade is not spinning.
At the very beginning when the guy who ran out of gas is being chased by Trevor Moorehouse wielding a chainsaw, he seems to be cutting branches off trees on the way, but you can plainly see the branches are just propped up and not attached to the trees. He just knocks them into is path with the chainsaw.