When the mine is dynamited, there is a shot showing the mine entrance. There is smoke and dust but little to no debris. However, the next shot of the entrance shows a large amount of dirt extending from the entrance as well as two large beams, none of which was shown earlier.
The town jail has a window in the first cell on the left which faces the street. However, that window is not visible when looking at the jail from the street.
After the dynamiting of the mine, Slim, Jess, and Calico view the mine entrance area from a secluded spot. They see the remaining two bad guys horses but not the bad guys themselves (Les, and Charlie). However, subsequent shots of Les and Charlie show that they should have been in plain view.
Les and Charlie (the two remaining bad guys at he mine entrance) see Slim, Jess, and Calico as they are riding towards the mine entrance. However, the good guys don't see them, but they should have as their attention was focused on the mine entrance and the bad guys would have been in plain sight.