Geheimnis im blauen Schloß

The mysterious voice that accuses the invited guests of their specific crimes is an uncredited Christopher Lee.

When Shirley Eaton dresses for dinner her underwear is white (as noted in another goof). She then puts on a dress before opening the door for Hugh O'Brien. Later in the movie, when she is locked in her room, she is seen undressing and wearing black underwear. This is not a goof or continuity error. This is the next day and she has obviously changed clothes. She is no longer wearing a dress, but a skirt and sweater.

The film was originally released with a "Whodunit Break" gimmick. Right before Ann is seen firing the gun at Hugh, the action pauses and an off screen voice tells the audience that the film will pause for one minute to give them a chance to solve the crime. The previous murders are shown under a large clock ticking the minute away. After the narrator tells the audience they can figure the mystery out if they have "paid close attention" he finishes by intoning "DO NOT BE SURPRISED BY WHAT YOU ARE ABOUT TO SEE!" At that point the action resumes. Some video and TV prints do not contain this fun "murder minute", but others (including the version shown on TV in the UK) still do.

Referenced in What's My Line?: Sam Spiegel & Hugh O'Brian (1966)