When Rachel and the woman she is chatting to in the bar say "F**k you, Anna Boyd" into Rachel's smartphone camera they say it at the same time, but when Rachel plays it back later their voices are not in harmony.
When Rachel first sees Megan and the therapist embracing, she gets excited and runs down the train to get a better look only to have another train going the opposite way obscure her view. When we see this scene from Megan's point of view we see only one train going down the track; the other train is nowhere in sight.
Rachel films on her cellphone in the horizontal way; when she views the recording it looks like it was done in the vertical way.
When Rachel goes to Scott's house and he gives her coffee, her coffee cup changes position on the table although she hasn't touched it yet.
When Anna is lying on the bed and Megan hands her the baby, the baby's mouth is empty. In the next shot, taken from the other side of the bed, the baby now has a dummy attached to a cord in its mouth. But in the following shot, taken from the original shooting position, the dummy and cord are missing.
When we see the train from the window of the white villa, passing on the track way beyond the garden, it is obvious that the distance is too great for anybody to be able to identify any faces at all - for instance the therapist on the balcony. The view as seen with Rachel's eyes from the train window, is greatly enlarged, as if it was seen though binoculars.
When Anna is trying to figure out Tom's Windows password, the password is visible. Windows obscures the password so that it's not visible.
When Tom, Anna, and Evie take the selfie in the beginning of the movie, they use the wrong type of camera and neglect to disable the rear flash.
The inside of the Metro-North train cars is not accurate.
The article describing the discovery of Megan's body says that Westchester Police SAYS they're investigating a murder case. Westchester Police is a plural noun so it should read Westchester Police SAY etc etc.