During the stagecoach ride, mention is made of Montana hoping to become the 41st state, to which Maverick is skeptical. In fact, while Montana did become the 41st state, it did so on 11 November 1889, only 6 days after North Dakota and South Dakota were admitted to the Union on 2 November 1889, and only 3 days before Washington. With 9 days between the 39th and 42nd states, the characters could not have expected Montana to have been the 41st state.
When Maverick and Mrs. Ross take the stage and it goes past a herd, there are four horses pulling the stage, however in the next shot when the stage pulls into town there are six horses pulling the stage.
After Maverick escorts Mrs. Ross to Montana, she hands him a few bills adding up to $1000 as promised in her contract. Maverick mentions that Lincoln was on those bills, but Lincoln was never on high dollar bills even back in the late 1800's. So, the few bills he received wouldn't have added to the $1000 fee.