At 01h11 30, when Mark's playing tennis alone, on the first shot, facing him, we can clearly see the playground with almost no tennis balls, when the camera comes his side, the playground is full of tennis balls.
Amanda states that her therapist diagnosed her with anti-social personality disorder (among others). No one trained in psychology would ever diagnose a teenager with this disorder because all teenagers, technically, fit on the spectrum. Also, neither girl is a true psychopath as both show hints of remorse at times.
Near the end of the film Lily arrives for a lunch meeting in a white Tesla Model S that includes added sound from a gasoline powered car shutting off with a distinct putt-putt-putt. The only sound that should have been heard other than the tires on the road is the parking brake clamping the back wheels.