This may hardly matter, but in New England recently I discovered how easily such popular myth turns into history. Few are so naive as to believe that George Washington said "Father, I cannot tell a lie," but how many know that Patrick Henry almost certainly never uttered the words "Give me liberty or give me death"? Nor is there any record of another American revolutionary, James Otis, saying "Taxation without representation is tyranny." And most of what we think we know of Paul Revere's ride was made up by the poet Longfellow.
One might be tempted to dismiss the Americans as a nation of easy fabulists, but I remember the distinguished Simon Hoggart pointing out that Harold Macmillan never actually said "You've never had it so good." What we "remember" is a headline-writer's compression of words roughly to that effect. As recently as 1979, Jim Callaghan was supposed to have returned from the Caribbean during the Winter of Discontent and asked: "Crisis? What crisis?", but that was an ever vaguer headline approximation of what he said. These errors may be traceable, but who polished up the mythical Casablanca and Star Trek "quotes" into their present form?