Revealing mistake: When Jen is at the cash bar and keeps putting money down, it's the same shot used multiple times since it's the same serial number on the $5. And in one shot they flipped the image to make it look different (you can see the "5" is a reverse image).
Revealing mistake: Becca attempts to break the window with a microwave, which she drops and it lands on her foot. However, the microwave doesn't appear to land on her foot at all. Becca's feet are positioned on the pavement next to the door, whereas the microwave is shown landing on the grass.