Revealing mistake: The saloon's upper level railing looks solid until just before the bad guy falls through it, when the cuts in the break-away section are prominently visible. (00:26:05)
Continuity mistake: Mona draws a heart in the dust on the bar and writes "Al + Mona" inside it. When we see a close-up it, however, the L is suddenly much larger than before, and the name "Mona" is written in smaller letters. (00:04:00)