From the screenplay by McMurtry and Ossana:
The tent is a little lopsided, but finally up. Ennis is trying to adjust a tent pole. Jack is sitting by the fire, playing a slightly damaged harmonica, squalling the old Tex Ritter rodeo tune, "Bad Brahma bull". A whiskey bottle sits next to him.
Ennis: "This tent don't look right."
Jack stops playing, glances over at Ennis and the tent.
Jack: "It ain't goin' nowhere. Let it be." (starts up again on the harmonica)
Ennis: (amused) "That harmonica don't sound quite right."
Jack: "That's 'cause it got kinda flattened when that mare threw me."
Ennis: "I thought you said that mare couldn't throw you."
Jack: "She got lucky."
Ennis: "If I was lucky, that harmonica would broke in two."
Both laugh.