In the airplane scene, the author of the book "Oh Brother, Where Art Thou?" is shown to be "Sinclair Beckstein", an amalgamation of the names of authors Upton Sinclair, Sinclair Lewis, and John Steinbeck.
In the scene where Sullivan is in bed with a cold the girl sits next to him on the bed, she shifts between being at higher level with him and then same level as him from shot to shot. (This happens more than once in same scene.)
When Sullivan and the "frail" jump off the train, each lose their hat. However, in the next shot, when both are on the ground the "girl's" hat is on her head
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.