Abacot: This word is said not to exist.
Generations of reference books once included this term, including the first edition of the Encyclopaedia Britannia, dated 1771: “the name of an ancient cap of state worn by the kings of England, the upper part whereof was in the form of a double crown”.
The word’s origin was a misprint in Edward Hall’s Chronicle of 1548. It was copied by Raphael Holinshed in his own Chronicles of 1577 and later by many others, including Nathan Bailey in his dictionary of 1721 and Noah Webster in 1828 (but not by Samuel Johnson in his dictionary of 1755, who was presumably saved from the blunder because the word was too rare and specialist to be included). A very few writers have used it in the reasonable belief that it was a real word, including George Augustus Sala, in his 1859 book Twice Round the Clock, or The Hours of the Day and Night in London, “The chandelier is of abnormous size, for any number of glittering festoons have been added to its crystal abacot.” (In case you’re wondering, abnormous is a real word, meaning irregular or misshapen.)
James Murray, the famous editor of the Oxford English Dictionary, found that the original word was bycoket, which was indeed a form of headgear, a cap or headdress with a peak both in front and behind, whose name he thought derived from an Old French term for a small castle crowning a hill. He triumphantly proved his case in an article in the Athenaeum in February 1882: “There is not, never was, such a word”. In the OED’s entry for bycoket, he described the perpetuation of abacot as “a remarkable series of blunders and ignorant reproductions of error”.
One may argue that since the word appears in dictionaries, and has — albeit rarely — been used, then it exists and ought to be treated as a proper word. If it were more common, that argument would have great weight (there’s no shortage of words that have been altered out of recognition through popular error), but as it has almost never been used, we may allow Dr Murray’s view to prevail.
Notwithstanding that argument, the word remains an awful warning to the writers of reference works who may be tempted to copy material from earlier works without checking their sources (worldwidewords.org).