The hand that pulls Nova Pilbeam's character Erica Burgoyne out of the hole in the mine scene was that of her future husband Pen Tennyson; they met on this film. They were married from October 19, 1939 until he was killed in a plane crash on July 10, 1941 at the age of 28.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
Though the film is officially based on the novel "A Shilling for Candles" by Elizabeth Mackintosh (writing under the name "Josephine Tey"), Alfred Hitchcock and his writers only used about one-third of the novel and changed the identity of the murderer.
The trivia items below may give away important plot points.
Though the viewer finds out at the end who the murderer of Christine is, the name of the murderer is never mentioned in dialogue. Even where Christine is shouting at her eventual murderer in a long conversation, she never names him, and the drummer at the end of the film is jokingly called "Old Man" by his fellow musicians, not by his real name. In the original story on which the film is based ("A Shilling for Candles"), the character has a name--Edward Champneis--but it is never given in the film. His name is given in the end credits as "Guy."