Continuity mistake: When Mr Henderson is dressed as a penguin in the hall and gives Billy an envelope to take to Mrs Henderson, it is manila (brown/yellow). When Billy sees Mrs Henderson in the control room, he hands her a white envelope.
When the killer is up in the radio tower, he tries to force the man up the ladder. He brandished a revolver and you can hear the hammer being cocked back. It shows the revolver more and you can see the hammer is still down.