You have to watch this scene to appreciate this blooper.
Wes has just gotten shot and Claire came running to his side. Wes tells Claire it's alright, not to worry.
Wes' second button on his shirt is buttoned.
In the next scene you see his second button on his shirt unbuttoned.
Then Claire mentions that Wes was going to take the Sheriff's job if it was still open. His second button is buttoned again.
The scene where Wes shakes hands with Lucas & Micah his shirt is unbuttoned again.
If you watch the close-up on this scene you see Claire fiddlin' with Wes' button on his shirt, it looks like Claire almost buttons Wes' shirt. Cowgirl!