No one will be surprised that swears and slang have shifted. Even aside from four-letter words, which are extremely hard to track through print, milder curses like "get the hell out" and "Jeez" are almost certainly out of place. There shouldn't be three characters who say "Yeah" (which most dictionaries don't have until 1905), for example. A more surprising counterpart, though, is that the language of politeness has changed in ways that we tend not to notice; a frequent category of errors in the script, apparently, are pat phrases like "Nice to meet you," "Could you please," and "Good to see you." Same goes for priggish language: Almost no printed books use George Pendleton's phrase "highly unusual" until 1900 or so.
Social changes have transformed the language of the country as well as the city. Lincoln refers to his father as a "smallholding dirt farmer" to emphasize his humble origins. Before 1890, though a "small holding" might refer to the land, but one would not speak of a "smallholding farmer" as Lincoln does. Much less a "smallholding dirt farmer," because while "dirt farmer" is an evocative phrase, it was essential unheard of in the period before farming stopped being the only way of life imaginable to most Americans. Young Mr. Lincoln, the other great movie about the president, makes this sort of mistake again and again. Spielberg and Kushner, though, don't spend as much time in nostalgia for rural America. The only similar mistake is having the Speaker of the House refer to a congressman's ""hometown" newspaper, a word that was very rare before most Americans lived in cities.
Lincoln is less successful in keeping out the language of politics and war. The Confederate olive branch, the movie's major subplot, seems to offer Kushner a minefield of modern language to stumble through. "Peace talks," his favorite, was used widely to describe negotiations involving the IRA and the PLO, but doesn't seem to have much history before Vietnam. "Peace plan" is almost as bad, while "peace offer" and "peace delegation" both appear only a few times before Versailles. These sorts of words are commonplace in any traditional history, so they should appear in printed language as well. Just to see how Americans actually described the affair, I checked 3,700 newspaper pages from the first four months of 1865 to see what words were actually used after "peace." "Peace mission," "peace interview," and "peace negotiations," none of which Kushner uses, would have been closer to the language of the time. Kushner's "peace proposal" was considerably rarer at the time than "peace proposition." About the only phrase that Kushner gets right in this subplot, in fact, is "peace commissioners."
The language of political equality has changed enormously as well. Even the phrase "13th Amendment" is out of place. At the time, people just said the "constitutional amendment" or the "slavery amendment": It had been 60 years since the last amendment, and no one was in the habit of numbering them. The same sort of mistake dogs the movie's discussion of racial equality. One particular character makes more than his share of this sort of mistake: the radical Congressman Asa Vintner Litton (Stephen Spinella, playing a composite character who seems most closely based on Henry Winter Davis). In one of the film's key scenes, Stevens refuses to state his belief in full equality to Congress in order to help the amendment on its way. Litton is furious: "You refused to say that all humans are, well... human!" But in 1865, referring to people as "humans" was slang, not an elevating way of being inclusive. Had a real Asa Litton wanted to express the notion of universal equality, he would have, like Thomas Jefferson a century before or Lyndon Johnson a century later, mentioned "all men;" even if he were being gender-neutral, he would have said "persons." In a similar vein, Litton and Ashley each talk about "racial equality" and "race equality" as the eventual goal, but the phrase would have been "Negro equality." Nowadays, that sounds like a completely meaningless difference, but actually, the difference between "Negro" and "racial" equality underscores just how adaptable American racism can be. One of the strangest results of "Negro equality" in Reconstruction was a short period when the California supreme court re-interpreted a law that prohibited blacks, Native Americans, and Chinese from testifying against white men: Thanks to the actions of the Radicals in Congress, blacks were now free from Chinese testimony as well.