Revealing mistake: When Dr. Pincus exits the hospital, as he approaches the brick walls, the shadow of the "ghost" nurse is seen in the sunlight on the ground as she gets into position at the edge of the right wall. You can also see her movement getting into position as she comes into the view of the camera. (00:13:45)
Revealing mistake: When all the ghosts wake up Dr. Pincus, he goes to the elevator where there are more ghosts, and in the shot facing Dr. Pincus the ghosts do not have a reflection in the mirror behind him. Odd though that they have reflections in windows - such as when the naked guy sneaks into Dr. Pincus's bedroom, countertops - such as when Frank sits at the bar counter with him, etc. (00:27:05)
Revealing mistake: After giving Richard the nitrous oxide, when Frank and Dr. Pincus walk down the street they pass under the apartment building's awning which has the convex mirrors, and Frank's reflection is visible as they walk by. (01:11:20)
Revealing mistake: In the auditorium, when Tea Leoni is giving her speech about Pepe the mummy, Greg Kinnear (who is not visible to anyone but Ricky Gervais) starts yelling and Ricky Gervais gets up to walk away. As Greg Kinnear is yelling, an extra a few rows up turns to look at him. That shouldn't happen because the whole plot hinges on the fact that no one can see or hear these people besides Ricky Gervais.
