Continuity mistake: When Platt is putting clapboards on the frame, after Tibeats tells him to make them flush he has one nail in the center of a plank when he sees Patsey being pulled out of another building. He takes his hand off the plank and watches. The plank is at three different angles depending on the shot, although Platt is standing several feet away and has not touched it again. (00:45:00)
Factual error: In the final scene where Solomon was reunited with his family, the doors he entered the room through had Edwardian style leadlight glass. The movie was set about 50 years before this style.
Continuity mistake: A man falls back into mud, in the pig pen. But his back is clean.
Factual error: In the close-up of the violin being strung, it is obvious the strings are of contemporary origin. You can see they are translucent plastic; not exactly historically accurate.
Continuity mistake: Towards the end while Mr. Parker speaks with Epps about Solomon's freedom the sun changes between shots.
Answer: Here's why, according to Northup in Twelve Years a Slave: "My great object always was to invent means of getting a letter secretly into the post-office, directed to some of my friends or family at the North. The difficulty of such an achievement cannot be comprehended by one unacquainted with the severe restrictions imposed upon me. In the first place, I was deprived of pen, ink, and paper. In the second place, a slave cannot leave his plantation without a pass, nor will a post-master mail a letter for one without written instructions from his owner."