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Continuity
When the murderer opens the envelope that came with the flowers outside the door of Nancy's apartment, he uses a knife to cut open the envelope. When he puts the card back in the envelope, he licks the envelope flap and closes it - it is not cut.
When Robert Manners (John Drew Barrymore, as John Barrymore Jr.) is watching Edward Mobley (Dana Andrews) on TV, he is clutching a copy of "Tales From The Crypt". When he drops it to the floor, a closeup of the comic book now shows it to be titled "The Strangler".
When the murderer opens the envelope that came with the flowers outside the door of Nancy's apartment, he uses a knife to cut open the envelope. When he puts the card back in the envelope, he licks the envelope flap and closes it - it is not cut.
The subway trains are Los Angles "Red Cars" and there is no third rail in the tunnel. There were no New York subways like that in 1956.
The chalk outline of the 2nd victim is in a physically impossible position.
There appears to be a knob for a inside-only bolt lock on Judith's door; it's above the main doorknob push-button lock, but there is no corresponding throw or plate on the edge of the door.
When Harry, Nancy, and Dorothy are in Nancy's apartment, the shadow of the boom microphone can be seen moving above the curtained entryway to another room.
When Ed and Nancy are walking upstairs to her apartment, the shadow of the boom microphone can be seen following them on the wall above.