Continuity mistake: The colour of John Wayne's shirt changes from black to grey after entering the burnt out building. It can not be dust, as the covering is far too even and complete, and a couple of shots later it is black once more.
Continuity mistake: At the very beginning of the movie, we see a train rolling down the hill, the first wagon has a dark door in the middle. In the next shot, we see the train coming in the station and the first wagon does not have a door on the middle part of it, but at the beginning of the wagon. Obviously another shot, another train.
Continuity mistake: During the election meeting held at the saloon, there's a large sign that reads "Capitol City Here We Come" on the wall by the stairs. We see this sign in numerous shots. When Valance enters the saloon there's still a sign on that wall by the stairs, but now the painted handwriting on this sign is different. Moreover, the original sign that had been by the stairs is now up on the side wall of the saloon, where Valance and his thugs stand; note the handwriting. (01:09:20 - 01:13:45)
Continuity mistake: Toward the end of the movie in the meeting hall, all the good guys are seated on the left and the bad guys are on the right, when looking from the stage. After John Wayne has his talk with James Stewart outside the meeting hall the doors kick open for Stewart to go back to the meeting. The good guys and bad guys in the audience are now on opposite sides of the meeting hall as you look toward the stage from the rear.
Answer: At first glance, it looks like the word "LEFT" in large, widely-spaced lettering. It's very unlikely that this would be a blocking cue, telling Jimmy Stewart to move left (which he does from that point). That's not the way blocking cues are done, for one thing, and a seasoned actor such as Jimmy Stewart wouldn't need such a cue. It's also very unlikely that it's a warning message from John Wayne who is hiding in an alley across the street, on Stewart's right. John Wayne didn't want anyone to know that he actually killed Valance, so he wouldn't alert Jimmy Stewart with any messages scrawled in the street. All hypothesis aside, it's probably just footprints in the dirt, an illusion of light and shadow.