When Sullivan and the Girl jump off the train and walk to the lunch stand, nothing is visible around the outside of the lunch stand--not a car, tree or anything. When Sullivan asks if the proprietor had seen a land yacht (a big RV), the proprietor points to the side and they look out the window and see the big land yacht parked there. Of course, if it had been there in the first place, Sullivan would have seen it right away and not gone into the lunch stand.
The first time all the men in the large homeless shelter are seen, a sign on the wall states 'Have You Written Home to Mother' with no question mark. The next time the sign is visible, the question mark is in place.
When Sullivan and The Girl are riding in his convertible, they drive under an overpass, but the light never changes even though the shadow is visible in the background.
When Sullivan is in bed with a cold and The Girl sits next to him on the bed, she shifts between being at higher level than him, then the same level as him from shot to shot. This happens more than once in the same scene.
When Sullivan and The Girl enter the giant communal sleeping room with the other tramps, the sign on the wall behind them mentions writing a letter to your mother. The sign is still there when the cook comes to wake them up, but in wide shots, the sign moves further left and a different sign is in its place.
John was sentenced for hitting the railway man with a stone. It seems this sentence would still hold even if they knew he was a Hollywood director. Sullivan was implying that the court would have treated him differently if they had known that he was an important person enjoying a high standing, which happens sometimes.
When the supposedly-unconscious Sullivan is being pulled into an empty boxcar by the bum who hit him, Joel McCrea is visibly pushing himself along with his foot, apparently because the other man is not strong enough to drag him.
The road numbers on the racing locomotive in the opening scene are reversed, indicating the film has been flipped.
When Sullivan is outside the boxcar door encouraging The Girl to jump out to him, the background scenery is unsynchronized. From the live-action shot taken outside, he is trotting along safely at about five miles per hour, but seen from inside, the scenery is moving behind him at breakneck speed, giving the illusion that Sullivan is being dragged on roller skates.
When Sullivan and The Girl are on and off the train, the sound and volume of the train going over the railroad tracks never change, indicating that the Sound Effects or Foley artist is looping the sound of the train tracks.
While in the whippet tank being chased by the coach, they pass a cop with a motorcycle, splashing him with clear water; when the coach passes him, there is a very quick shot of him splashed with more clear water, but in the next shot, he is miraculously covered in mud.
When Sullivan and The Girl, disguised as hobos, walk along a moonlit river, a stagehand's legs are clearly seen from the knees down in a tree above their heads.