Revealing mistake: When Abby and Jen are drinking on the dock, Abby suddenly falls into the ocean and drowns. Just as they show a close-up of Abby falling about to hit the water, you can tell it's a stunt person. Monica Keena is a fairly short actress, and the stunt person is noticably taller with longer legs. Also, Abby had been wearing a very decorated black and silver skirt while on the dock. But the person about to hit the water appears to be wearing dark clothing without any silver or glitter.
Revealing mistake: When Rachel and Adam first meet, their cars crash in the car park. In the shot where they both get out of their cars, you can see a dry patch of ground under Rachel's car, but the rest of the area is wet. Why would that space be dry, if Rachel just crashed and it isn't raining heavily at the time?