Revealing mistake: The murder victim's skeleton, which Rusty finds in the attic, has a clear cut all the way around its skull. It also has metal pins holding together what turns out to be rubber bones: one of the ribs wobbles up and down when Rusty brushes against it with the cash box.
Character mistake: Charlie tells Thedy that the hair ribbon embroidered with her name cost 65 cents, "five cents a letter." But her name is Thedy Sue Hill, which is only 12 letters - or 60 cents worth. (And no, the extra is not due to a sales tax, which in 1962 would have been two cents, tops, and didn't even exist yet in many states.).
Continuity mistake: In the dining room, Mary places the telephone on the kitchen counter by reaching through the double doors over the bar. The doors, which she pulls shut, bounce back open slightly on their own. Mary then commits suicide. No one else has been there, but when Agatha returns to discover the body, the bar doors are tightly shut again.