When we first see Yosemite Sam, he has a wart on his nose. At some point in the cartoon, the wart disappears, never to be seen on Sam again.
The Mail car starts out as gray over brown, with no climbing rails. When Yosemite Sam rides up to it, the bottom is blue-gray, and there are now climbing rails on it.
When Yosemite Sam hands his drawing of a gun to Bugs, the page folded over the back just switches sides - the pad is never flipped around.
The gun that Yosemite Sam uses never runs out of bullets or gets reloaded.
When the red ink is spilled all over Yosemite Sam, not one drop ends up hitting the floor.
Any time Yosemite Sam shoots his gun at the floor, the walls, or at the door no bullet holes ever appear.