Nell is smiling in the picture that the man takes with his phone, but when the photo was taken she was not smiling.
When Nell asks what the other girls are watching on the YouTube video, the first initial scene where the girls turn the monitor to show her, the caption for the video says "Camera found in woods". However, when the camera zooms in on the video, the caption now reads, "Crazy possessed girl in woods"
When Nell wakes up and Gwen is gone, her alarm clock is flashing making it seem her encounter during the night caused it to lose power. In the next shot, it is a longer view of the room, and the alarm clock is completely off.
When Jared leaves the bathroom and goes into the kitchen, the blankets are alternately off/on the bed between shots.
Closing scene, Nell starts a car, dash clock is 8:13. 30 seconds later, focus is on the clock again, now it reads 8:06.
As the front bike wheel turns in a Segway scene, it makes a clicking noise as if the back wheel were turning.
The movie begins with Nell showing up in a stranger's house alone. How she escaped from the cult that was holding her captive at the end of the previous movie is never addressed or even questioned.