While playing pool in the bar, the balls make a futuristic electronic sound when hit. However, later in the brawl when someone gets thrown onto a pool table, they make the traditional sound.
When Badger shows up with his armed men, a couple looking at a shop window in the background turn, see the men, and hurry away. The camera cuts away then back to Badger, and the same couple are there again, repeating the same actions.
During the duel, Atherton slices Mal's right arm just above his bicep. After the duel when Mal and Inara return to the crew, Mal has blood on the front of his left shoulder but none where he got cut.
At a number of points during the duel, the 'mist' in the field can be seen being pumped out from behind the tree, not drifting as it should.
There is a wide shot of the ballroom dancers and in the archway in the back of the room you see the director walk into view wearing a Hawaiian shirt, shorts, and a baseball cap with headphones down around his neck. Just as he raises his hands to speak to the dancers, they cut away.
When Serenity approaches Persephone (in the "dirt kissing" scene), you can see a reflection of a person in a white T-shirt in Serenity's windshield who is not regular crew.