Nicholas Cage: That's not true. Do you know where the expression "his name is mud" comes from?
Comic relief: Um, does anyone but you?
Nicholas Cage: It comes from Dr. Samuel Mudd, the guy who treated Booth's broken leg. History remembers him with that phrase.
As for "his name is mud," there's an old story that the expression derives from Dr. Samuel Mudd, who unwisely took pity on Abraham Lincoln's assassin, John Wilkes Booth. Mudd treated the broken ankle Booth suffered in his leap to the stage of Ford's Theater; for his trouble, he was sentenced to life in a federal prison. But Mudd isn't being commemmorated in "his name is mud." The phrase first appeared in print in 1820, 45 years before Lincoln's assassination. It probably originates in another obscure bit of English slang — "mud" was an eighteenth century equivalent of our "dope" or "dolt" and was used through the nineteenth century by union workers as a rough equivalent of "scab."
As soon as I heard Nicholas Cage utter this statement, I figured it was inaccurate. A cursory Internet search would have told someone on the movie set it wasn't true, and the line wasn't at all integral to the story. I always wonder why someone decides to go ahead with something like this. (We did like the flick, though, for what it was: an action movie with a little history thrown in, and something we could watch as a family.)
It's a comment by a character. Good characters should be like people – fallible.
So I don't have a problem with Nicholas Cage's character giving a false etymology; it's what real people do.
Well, I'd have little-to-no problem with this if he weren't presented as a student of American history, or if they did something to deflate his authority a bit. In the world of the movie, he hasn't made a mistake.