When Nikolai is about to stab Jennifer, there is a distant shot where he says, "We have to make a really good impression on him," Jennifer isn't crying and her mouth is closed.
Nikolai says, as the band is about to perform the ritual: "We got a waxing moon. Perfection."
Then we get cut to a shot of a moon that's visibly shrinking, while waxing means growing moon.
When Jennifer is lighting her tongue with the lighter, her hair is front of her ear in distant shots, then behind her ear in the close-up.
The first time Needy and Chip walk away from her locker, the shoulder straps of Chip's backpack jump from one shoulder on, to two, to one, to two again.
Just after the fire in Needy's kitchen, Jennifer appears in profile, with a ghastly, toothy smile. In the next full-face shot, her mouth is closed, and the ghastly smile appears slowly.
Needy does research on the occult and determines that Jennifer is a succubus, a demon which feeds on flesh. A succubus is believed to be a female demon that has intercourse with sleeping men, and does not, in fact, eat flesh.
After Melody Lane catches on fire and Needy and Jennifer are outside kneeling in the parking lot it is obvious that there are no lenses in Needy's classes as there is no reflection of the fire.
When Jennifer is in the forest with Jonas, as she unzips her jacket, it can clearly be seen that she has no belly button, though it is shown in other scenes.