The document containing evidence of the missing link in your research invariably will be lost due to fire, flood, or war.
The keeper of the vital records you need will just have been insulted by another genealogist.
Your great, great, grandfather’s obituary states that he died, leaving no issue of record.
The town clerk you wrote to in desperation, and finally convinced to give you the information you need, can’t write legibly, and doesn’t have a copying machine.
The will you need is in the safe on board the “Titanic”.
The spelling of your European ancestor’s name bears no relationship to its current spelling or pronunciation.
The ancient photograph of four relatives, one of whom is your progenitor, carries the names of the other three.
Copies of old newspapers have holes which occur only on last names.
No one in your family tree ever did anything noteworthy, always rented property, was never sued, and was never named in wills.
You learned that great aunt Matilda’s executor just sold her life’s collection of family genealogical materials to a flea market dealer “somewhere in New York City.”
Yours is the only last name not found among the 3 billion in the world-famous Mormon archives in Salt Lake City.
Ink fades and paper deteriorates at a rate inversely proportional to the value of the data recorded.
The 37-volume, 16,000 page history of your country of origin isn’t indexed.
The critical link in your family tree is named “Smith.”