In the realm of word histories, I've mentioned this error before (in an answer to a question about Preventative vs. preventive). Merriam-Webster's Webster's Dictionary of English Usage (1989) has this commentary on preventive versus preventative:
preventative, preventive The critics have panned preventative for over a century, preferring it shorter synonym preventive in spite of the fact that both words have been around for over 300 years and both have had regular use by reputable writers. Here is the basic premise behind the objections: if two similar adjectives are derived from the same verb, then one of them must be in some way inferior to the other, and the likely culprit is the longer one. But the only real difference in status between these two words is that preventative is much less common than preventive. If you decide you like the sound of the extra syllable, and are willing to brave possible criticism for it, you may take heart from the example set by these writers: [examples from the works of Daniel Defoe, George Washington, Henry Wallace, Oran Brown, Frederic Wertham, and Michael Stugrin omitted].
You may wonder how preventative came to be objected to. The earliest attack is in [Richard Meade] Bache [Vulgarisms and Other Errors of Speech] 1869; he said that there was no such word. Bache's book was one of those used by [Alfred] Ayres [The Verbalist] 1881, who also criticized preventative. From Ayers it went to [Frank] Vizetelly [A Desk-Book of Errors in English] 1906, [Ambrose] Bierce [Write It Right] 1909, [H.N.] MacCracken & Helen] Sandison [{Manual of Good English] 1917, and [Charles] Lurie [How to Say It] 1927. [Henry] Fowler [A Dictionary of Modern English Usage] 1926 picked it up too, and so it has gone right down to the 1980s. A couple of commentators—[Bergen] Evans [Comfortable Words] 1962 and [William] Watt [A Short Guide to English Usage] 1967—realize that preventative is acceptable. Probably none of the recent objectors realizes that his opinion goes back to Bache 1869. But the moderns do not claim that preventative is nonexistent anymore; nowadays they say it is wrong because it is "irregularly formed." That is not so, of course. It is formed in just the same way as authoritative, quantitative, normative, talkative, and other words to which no one objects.
Despite the painstaking inquiry that Merriam-Webster's researchers made into the origin of criticism of preventative, they somehow failed to notice this entry for preventative (which appears in small type between the entries for preventable and prevented in the 1847 edition of Merriam-Webster's An American Dictionary of the English Language:
[PREVENTATIVE is a gross blunder.]
The 1857 revision of this edition of the dictionary takes the remark about preventative out of brackets and moves it to a more sensible location—the end of the entry for preventive, but it doesn't change the wording. The next new edition of the American Dictionary of the English Language (1864) and the first edition of Webster's International Dictionary (1890) have this entry for preventative:
Preventative, n. That which prevents ; — incorrectly used instead of preventive, q.v.
James Martin, in his pre-Bache book, The Orthoepist: Containing a Selection of All Those Words of the English Language Usually Pronounced Improperly (1851), gives due credit to Webster's for the objection:
PREVENTIVE, a. Tending to hinder. [Preventative is a gross blunder.—Webster.]
And Charles Northend, Exercises for Dictation and Pronunciation (1872) repeats Martin's wording without any mention of Bache.
So whatever sins may justly be set in the record against Richard Meade Bache, being the originator of published hostility toward preventative is certainly not one of them.
