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.
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.
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.
The goof item below may give away important plot points.
When Rachel sees the article about Megan's disappearance/murder her age is listed as 27 however, at the end her grave stone says "1990-2016" making her 25 or 26 depending whether she died before or after her birthday.