Claim: The modern flush toilet was invented by a 19th-century British plumber named Sir Thomas Crapper.
Analysis: Thomas Crapper (1836-1910) did exist, and was a plumber, and is, in fact, credited with improving the functionality of the early flush toilet (or "water closet," as it was then called), but he did not, contrary to popular belief, invent the pseudo-eponymous bathroom appliance from scratch.
Credit for that goes to 16th-century author Sir John Harington, who not only came up with the idea but installed an early working prototype in the palace of Queen Elizabeth I, his godmother. The first patent for a flushing water closet was issued to Alexander Cummings in 1775, sixty years before Thomas Crapper was born.
The son of a Yorkshire steamboat captain, Tom Crapper's destiny was fixed when he was apprenticed to a master plumber at the age of 14. He owned his own plumbing shop in London by the time he was 25. Crapper was awarded nine patents for plumbing innovations during his lifetime, three of them consisting of improvements to the flushing water closet.
Though he made his name as a sanitary engineer to bluebloods, Crapper himself was lowborn and never knighted, so it's a mystery why storytellers consistently award him the title "Sir."
Compounding the error, he is sometimes referred to as "Sir John Crapper."