The Gentleman from New Orleans - S5-E18
Revealing mistake: They are trying to find where Jean Lafitte is hiding. Thinking that he is behind the drapes which cover a window, two bullets are fired into the drapery. The bullets should have shattered the window, but when they look behind them (and he is not there) the window and the frame are untouched.
The Saga of Muley Jones - S5-E26
Revealing mistake: The Cartwrights' second cousin shows up and sings a lot. His singing breaks glass all over town. 1/4 of the way through the episode, while at the ranch, Ben is about to serve tea or coffee while Huley is singing upstairs. As Ben goes to pour it into a cup, the pottery kettle shatters but no tea or coffee or water comes out of the pot, showing it was empty.
