When the Danbys come into town for the final showdown, there are 13 of them. During the battle, two are shot down by Prudy, yet when they all come to the cannon at the end, there are still 13 of them.
When Prudy is grabbed in the butt by one of the fighters during the mud fight, the exact same scene had been shown from a distance a few seconds before. The scene is used twice.
Before the final shootout, thirteen Danbys are shown riding into town and dismounting. There is a cut to two men walking towards the sheriff's office and when the camera cuts back, all the horses have now disappeared.
During the final gun battle, the intact portions of the glass windows that Prudy has broken change from shot to shot.
During the mud fight at the beginning of the movie, those fighting switch between being covered in mud and nearly mud-free.
Jake uses dynamite to fish, lighting a fuse to set it off. Dynamite cannot be detonated in this fashion- it requires a blasting cap, which is fired by electricity.
When the sheriff first goes to the new jail, he finds a bucket of red paint and yet nothing in the entire office had been painted red. The cell blocks were brick and the walls in the office were yellow.
During the fight scene in the mud, you can clearly see some of the fighters missing each other by a wide margin, but the intended target still falls down.
During the fight at Emma's, a man gets a plateful of food mashed into his face. The "metal" plate folds like a paper plate.
After the Sheriff leaves the red paint disguised as blood on the floor to discourage jail breaks, he leaves the bucket of red paint and brush by the cell, thus exposing the illusion.
During the mud fight scene, when Jason and the town council use the 'fire engine' to break up the fight, there is no water source and the engine has no water tank.
When clarifying the job as Sheriff, Jason says "one-hundred fifty dollars for room and board" instead of "one-hundred fifty dollars *plus* room and board".
While in the hardware store, the Mayor covers the hole in a metal washer with masking tape so that Jason McCullough can demonstrate his shooting skill. The movie appears to be set in the 19th century and adhesive tape was not invented until 1925.
When Prudy is counting the number of the Danbys who will come to town, she calls Walter Brennan "Milt Danby". Pa's name is never given, but Milt Danby is said to be Pa's brother. She goes on to tell Jason that Pa's other brother, Ira Danby, has three boys and Milt has two. However, in the jailbreak attempt scene, Pa Danby tells Luke Danby to ride to Milt's and get Steve, George and Little Milt, and he tells Tom to ride to Ira's to bring back him and his two sons, Billy and Jack. She switched the number of children for each brother.