Betty Hutton sang songs that had not been written. Frankie and Johnny wasn't written until 1904 and She'll Be Comin' Round the Mountain in 1899. Little Brown Jug was accurate for the time. It was written in 1869.
When Marshall Dillon throws the saddle at Claude Akins character knocking him to the ground and then picks him up and hits him, the shot is so close that it's obvious It's a much younger man, not Claude Akins.
At the end, during the quick fight between Matt and Sherne, it is a very obvious stunt double for Sherne. No camera tricks, and no attempt by the double to hide his face.
In the opening scene, the camera is positioned on top of the stage coach for a different POV. At one point, the camera shadow is seen on the back of the man to the left of the stage driver.
When Molly is shooting the pine cones that have been placed on a branch, each one gets blasted and the one next to it shakes enough that it should have fallen. Most likely, they were all wired with squibs to simulate being shot.
Molly is so busy working at least several nights in the Long Branch, then spending weeks learning how to shoot, that she appears to have abandoned her young son. She checks on him once while he is sleeping, then he doesn't show up again until the end of the story. Even then, she doesn't even acknowledge him until Matt strongly suggests that she take her son and leave on the next stage.