Question (if you haven't seen the DVD, here be spoilers)

Why the heck did they choose to show Courtney (gas station chick) on the DVD cover holding a knife? As soon as I saw her in shorts I recognized her from the back of the cover of the DVD. And I knew that she was going to turn out being the murderer.

The cover can be seen here:

http://www.covershut.com/covers/7-Below-2012-Wide-Screen-Front-Cover-6 6566.jpg

Wasn't this supposed to be the twist of the movie?