Revealing mistake: When Marion is driving her car through the night, she keeps looking at her speedometer. You can clearly see that the gear shift lever is in "park" as she is driving.
Revealing mistake: If you look closely, you can see the strings moving the Ouija board planchette while it's floating in the air towards Medea. (It's easier to spot them on the high-definition Blu-Ray version, but they can be faintly seen in other versions of the film if you know where to look).