Faith picks up and holds the broken painting in one shot. Then, in the next, she runs towards it lying on the ground as if she had never picked it up.
Right after we see John talking to Vicky in her small shorts, we see John near a railroad and you can clearly see his breath a couple times.
Martha's hair while the three girls are talking by the fire.
The cult members only accuse female characters of being the Incubus. In folklore, however, an Incubus is an exclusively male demon, the counterpart to the exclusively female Succubus.
While Martha is taking a bath, supposedly naked, a black bikini is clearly seen through the bath bubbles.
In the opening sequence, when William breaks Faith's painting and throws it, the painting is clearly attached to wires of some sort.
When Martha is changing into her nightgown and William is watching her through the window, she isn't wearing any underwear. She slips on the nightgown and sits down to brush her hair when the camera switches to William outside. When she stands up a minute later to go to bed, she is clearly wearing underwear.