I’ve been meaning to post this one since I first saw the MPSIMS thread, and this thread and Derleth’s reply brought it to mind.
From circa 1530, reported by Erasmus:
While saying the Latin Mass, an elderly priest perpetually used the incorrect word “sumpsimus” instead of the correct “mumpsimus”. When a younger priest tried to get him to change his practice, he said, “I’ve been saying it for 40 years. I’m keeping my old mumpsimus, and you can take your new sumpsimus out of here.”