During the opening gunfight, the telegraph office windows are shot out. Just a little while later, they're all unbroken.
When the gunsmith is fitting Wes for a new rifle, he is holding the stock from a model 1898 Mauser, which would not be invented for another 20 years. Wes also picks up a Winchester and looks through the barrel to see the lady gunsmith, which is not possible due to there being no straight line of sight through the action.