Mumpsimus: Stubborn insistence on an incorrect usage (e.g., "between you and I" or "nucular") even after being proved wrong.
Mumpsimus is an example of what it describes: according to World Wide Words, the word was accidentally coined by a medieval monk who persisted in saying a phrase in the Latin Eucharist incorrectly, "either because he was illiterate and had learned it that way or because it had been transcribed incorrectly in his copy":
What made this particular mistake memorable is what the monk was supposed to have said when he was corrected. According to the version of the incident told in 1517 by Richard Pace, later the Dean of St Paul’s Cathedral in London, the monk replied that he had said it that way for forty years and “I will not change my old mumpsimus for your new sumpsimus”.
As a result, the word came to be applied to someone who sticks obstinately to their old ways, in spite of the clearest evidence that they are wrong. The word can also have the related meaning of some custom or notion that is adhered to, even though it has been shown to be unreasonable.
Over the centuries sumpsimus came to have an independent meaning of "persistence in using a strictly correct term, as a rejection of a more common but erroneous term." In a column for the San Francisco Chronicle that started out lamenting a failed sump pump and meandered over to sumptuary and sumpsimus, Jon Carroll wrote that "There's a lot of sumpsimus around 'enormity' right now."
That's because the sticklers hew to a dictionary definition of enormity—"monstrously wicked"—while more casual writers and speakers use enormity interchangeably with enormousness. In fact, enormity and enormousness were synonymous until the 19th century. Read what World Wide Words has to say about President Obama's use of "the enormity of the task" in his victory speech last November.