Carey Mulligan has a noticeable and distinct mole on the left side of her face, next to her lips. Around the 1 hour 5 minute mark, when Mulligan's and Burnham's characters get back together in the coffee shop, it is apparent that the image is reversed, as the mole now appears on the right side of her face.
As Cassandra walks up the sand track dressed as a nurse it is clear that her socks get soiled. As she lies on the bed her socks are fairly white again.
During Cassie and Ryan's conversation in the diner, the level of cola in Cassie's glass changes between shots.
During the dialogue between Cassandra and Jordan, as the scenes change from one to the other, at a point, the height of the pillow behind Jordan and the position of the towel changes and then it gets back to the initial state in the next scene.
After meeting Cassie's parents, Ryan and Cassie are at his apartment, on his bed. He comes up on his left elbow, takes her right hand in his right hand before making a declaration. Point of view changes and her hand is resting, without his near, across her body.
A Union Pacific train is seen crossing the road where Cassie has stopped. Union Pacific operates in 23 western US states, but not in Ohio where the movie takes place.
Medical students would most likely know that a log fire wouldn't be strong enough to completely cremate a body and it certainly wouldn't have been reduced to ash to the degree that is shown when the police were searching the spot. And if it could have been made hot enough, it would have melted or at least damaged the necklace.
Even on the highways, there's no traffic.
The numbering of the 'events' are roman numerals, well so it would seem to start with, but in fact they are groups of five as in tally marking, it's just that they use the capital letter I...
The feet of the white knee-highs are noticeably dirty when swapping sneakers for red heels before walking without shoes on the unpaved road.
Cassie has a mole on the left side of her lips. When Cassie and Ryan kissed each other for the fist time (around 1:05:35), mole is in the right side, showing us a mirrored image.