Opening scene when the stage is pulled up by robbers one robber jumps of stage seat to the ground behind West twice.
Arty takes a keg of black powder and pours it out on to the bed of a wagon, then pours a trail away from the wagon in lieu of a fuse. But as he pours the "fuse" he fails to pour it evenly, and instead pours it so that the line is broken in multiple places. Nonetheless, when Arty lights the fuse, it burns right up to the wagon. And even though the lighted powder fuse has no way to rise to the wagon bed, it defies gravity and blows it up anyway.
Nighttime scenes were filmed using a filter to darken scenes. BUT this technique leaves the sky blue when in fact the night sky is always black! This technique tends to also leave landscapes bathed in sunlight, highly detailed, evident, when in fact it should, fade, disappear, into the shadows and darkness of the background of the night.
Artie's "piano playing" is completely unrelated to the dubbed music. In addition, the keys on the piano do not even work - they are fixed and do not depress.
Cassidy fires a shot, knocking over a clock in a shop window. As the clock falls, a face appears reflected in the window glass, looking like two faces because the glass is cracked.
In the opening sequence, when the coach is shown from the front, the tire tracks of the vehicle filming the action are plainly visible in the dirt.
In the opening sequence, when the guard jumps off the coach, the area he jumps into is plainly not grass - obviously prepared so the stuntman could land safely.