The train conductor fires his shotgun at least ten times in the scene in the tunnel where everything goes black. The shots were far too close together to allow for any reloading and, since he was carrying a double barrel shotgun, that many shots would not have been possible in that span of time.
When Martin Balsam is getting dressed after being interrupted during his dalliance with Tina Louise, his collar is unbuttoned and vertical. When he is wearing his dressing gown, the collar is covered by the gown. However, when he steps outside, the collar is visible again.
When a shotgun is fired during a shootout in the street, both the report of the gun and the bullet hole in the wall are from a rifle.
When McKay pulls the chain on the toilet, it's on the left side of the commode. Almost every toilet of the period had the chain on the user's right, because most people were right handed. When handles replaced the chain, the plumbing wasn't moved to the other side, which is why almost every modern toilet has the handle to the viewer's left.
During the chase after the train, a wagon with two men aboard appears to crash into a tall tree stump and overturn. The wire and bracket attached to the wagon to pull it over are clearly visible.
When Bump (one of the train conductors) fires his double barrel shotgun, only one projectile seems to come out (only one hole is made: either against a wooden wall or in the stove pipe). A shotgun fires many pellets from a shotgun shell that would create a wide blast pattern when it hits a wall (or pepper a stove pipe with many pellet holes).
When Mary fires a double-barrel shotgun, the sound effect is a rifle and a single bullet is shown hitting a post with a ricochet sound.