Revealing mistake: In the shot of the helicopter hovering above the apartment building, the blue screen technique is needlessly revealed by the eratic and physically impossible movements of the chopper. This could have been avoided if the camera shooting the aircraft had been stationary. It appears, however, that it was a handheld shot.
Revealing mistake: Just before Anthony Dawson is going to be stabbed to death by Grace Kelly with a pair of scissors, you can see for a very short moment that the scissors are already attached to his back.