The car Clara and Chet drive up to the cliff is not the one that goes over and crashes on the sea-side rocks below - it's an older make and model (and much less expensive for the studio).
As Uncle George is trying to wake Rodney out of his hypnotic state, he snaps his fingers. Rodney does not wake up, so Uncle George prepares to slap Rodney's face. Rodney's eyes are closed, but right before Uncle George slaps him, Rodney flinches in anticipation of the slap.
When the doctor has Rodney under hypnosis, he asks him, "Why did Clara try to blackmail you?"
The doctor wouldn't know anything about a blackmail because Rodney only stated that Clara showed up.
When Clara and her accomplice are pushing the car off the cliff near the beginning of the film, the movie lights are reflected in the chrome bumper.