Houdini had a session with Mrs. Doyle where she supposedly contacted the spirit of his deceased mother. She wrote a long, emotionally moving message, supposedly under the influence of his mothers spirit. However, Houdini was not convinced.
First, the message written by Mrs. Doyle called him by the name "Harry." His mother, Cecilia Weisz, never called him by the name "Harry," always by his given name "Erich," which, in his mother's native language of Hungarian or German, might sound like "Air-eeh" to a native English speaker.
Second, the message was closed with the sign of a cross. Cecilia Weisz was a devout Jew. If she would have signed a message with any religious symbol, it would have been a Star of David, not a Christian Cross. Further, she was never known to have signed any letter or message in that way while she was alive.
Third, and finally, the message never contained the word "forgive," a secret code word arranged between Houdini and his mother before her death. Without that code word, Houdini would have never considered the message to be genuine. In fact, as I remember the story, this was probably the greatest source of friction between Houdini and the Doyles. While the Doyles insisted that the message from Cecilia came from the Great Beyond, Houdini knew that it could not be genuine and was upset that anybody, knowingly or unknowingly, would deceive him in such a way.
As to the Cottingley Fairies, the way I understand, those photos were never meant to be considered evidence of the existence of fairies. They were just two young girls playing around, having fun. They were never meant to be made public. The first ones were kept in the family for a couple of years before Sir Arthur Conan Doyle and others heard about them and publicized them.
I believe that, like Abagail Williams and Betty Parris, the two young girls who made the first claims of witchcraft in Salem, Massachusets in 1692, once such a famous and respected person such as Sir Arthur Conan Doyle proclaims to all that the pictures are genuine, Elsie Wright and Frances Griffiths felt compelled to say that the fairies were real.
In fact, as I have read, this is exactly what the Elsie said in her 1985 interview: "Two village kids and a brilliant man like Conan Doyle – well, we could only keep quiet." Frances was quoted as saying, "I never even thought of it as being a fraud – it was just Elsie and I having a bit of fun and I can't understand to this day why they were taken in – they wanted to be taken in."
Personally, I think that two young girls took some pretty neat photographs. They are imaginative, creative and fun. Considering the state of the art in 1917, they are pretty darned good. If I had kids who took photos like that, I would be pretty proud of them...
... but I would never even consider them to be pictures of real fairies.