In her intriguing book The Science of Sherlock Holmes, E.J. Wagner recounts the true tale of Sir Roger Tichborne.
In 1854, Sir Roger was reported as lost at sea. His mother refused to believe that her son, whom she had lovingly raised in France, was gone forever. She kept putting out inquiries, asking for any news on her son.
Twelve years after the loss of Sir Roger, it appeared that Lady Tichborne's prayers had been answered. She received a letter from Australia (from a lawyer) claiming to have found her son. The letter explained that having been shipwrecked, Sir Roger eventually made his way to Australia, where he became involved in a series of business ventures after having vowed to make a success of himself following his miraculous escape. Unfortunately, the businesses did not work as well as he had expected, and he had been too embarrassed to contact his mother.
However, he had recently seen her inquiries and was filled with remorse for the worry he had caused her over the years! The letter concluded with a request to send the money for the travel fare for Sir Roger, his wife and children. Lady Tichborne was delighted to hear this news, and sent the relevant monies to allow for the family reunion. When Sir Roger arrived in England he was received by Lady Tichborne as her long lost son, and granted a stipend of £1,000 p.a.
However, not all the Tichborne family were quite as convinced that this new arrival was indeed the real Sir Roger. After all they reasoned, Sir Roger had been a lithe man of slim frame, the new arrival was obese in the extreme.
While people can change their size, it is rare that tattoos disappear. Sir Roger had some, the new arrival had none. Nor is it easy to change one's eye colour, Sir Roger had blue eyes, the new arrival had brown eyes. He was also an inch taller than Sir Roger had been, didn't speak French (which Sir Roger did) and had a birth mark on his torso which Sir Roger didn't!
Somehow Lady Tichborne managed to ignore all this evidence.
It was only after her death, that the family finally managed to show that the Australian import was an impostor. He ended up serving ten years for imposture and perjury.
H/T James Montier