H. Con-172 - S3-E11
Character mistake: Bartlet tells Leo that Jackson's censure was expunged in 1836, but it was actually January, 1837. (00:41:10)
Character mistake: The President refers to Beowulf, saying that it was written in Middle English. He's mistaken; it was written in Old English, or Anglo-Saxon.
The U.S. Poet Laureate - S3-E17
Character mistake: Jed Bartlett is talking to Leo and Hoynes. He says 'Calvary' instead of 'Cavalry' in the line "Tell Josh that General Pulaski was a Polish Brigadier General who vanquished the Russian and Prussian military then came to the colonies and commanded our cavalry during the American revolution."
Chosen answer: A really annoying innate lack of sense. My friends and family say the same thing all the time, and I'm endlessly trying to correct them. I think people just don't know any better and (ironically) couldn't care less that they're speaking incorrectly.