Continuity mistake: Right at the end when the man takes his mask off, a strand of blonde hair hangs out from the back of "his" head. In the next shot the hair is dark brown.
Audio problem: When Miss Jessica Marbles is reading and "finds out" who the murderer is, she closes the book and puts it down saying, "good heavens." But her mouth does not move.
Lionel Twain: You've tricked and fooled your readers for years. You've tortured us all with surprise endings that made no sense. You've introduced characters in the last five pages that were never in the book before. You've withheld clues and information that made it impossible for us to guess who did it. But now, the tables are turned. Millions of angry mystery readers are now getting their revenge. When the world learns I've outsmarted you, they'll be selling your $1.95 books for twelve cents.