When it's a hoax.
The bathtub hoax was a famous hoax or practical joke perpetrated by the American journalist H. L. Mencken, involving the publication of a fictitious history of the bathtub.
On December 28, 1917, a parodical article titled “A Neglected Anniversary” by H. L. Mencken was published in the New York Evening Mail. It claimed that the bathtub had been introduced into the United States as recently as 1842 and in England as late as 1828. The article went on to describe how the introduction of the bathtub initially was greatly discussed and opposed, until President Millard Fillmore had a bathtub installed in the White House in 1850, making the invention more broadly acceptable.
The whole article was entirely false, but was widely quoted as fact years later, even until the present day. In 1949 Mencken wrote:
The success of this idle hoax, done in time of war, when more serious writing was impossible, vastly astonished me. It was taken gravely by a great many other newspapers, and presently made its way into medical literature and into standard reference books. It had, of course, no truth in it whatsoever, and I more than once confessed publicly that it was only a jocosity... Scarcely a month goes by that I do not find the substance of it reprinted, not as foolishness but as fact, and not only in newspapers but in official documents and other works of the highest pretensions.