From some errant wiki-browising:
Houdini, chronicled his [Spiritualist] debunking exploits in his book, A Magician Among the Spirits. These activities cost Houdini the friendship of Sir Arthur Conan Doyle, the creator of Sherlock Holmes. Conan Doyle, a firm believer in Spiritualism during his later years, refused to believe any of Houdini's exposés. Conan Doyle came to believe that Houdini was a powerful spiritualist medium, had performed many of his stunts by means of paranormal abilities and was using these abilities to block those of other mediums that he was 'debunking' (see Conan Doyle's The Edge of The Unknown, published in 1931, after Houdini's death).
This disagreement led to the two men becoming public antagonists. Before Houdini died, he and his wife, Bess, agreed that if Houdini's spirit came back to earth, he would utter "Rosabelle believe" as a secret codeword to prove that it was actually him. This was a phrase from a play that Bess performed in when the couple first met. Bess Houdini, the magician's widow, held yearly séances on Halloween for ten years after Houdini's death, but "Houdini"'s spirit never appeared and communicated the passphrase. In 1936, after a last unsuccessful séance on the roof of the Knickerbocker Hotel, she put out the candle that she had kept burning beside a photograph of Houdini since his death, later saying in 1943 that "ten years is long enough to wait for any man."