This slip up can be found in the scene right before the New Year's Eve dance when Chavez is demonstrating his knife throwing abilities for the regulators. He throws three knives into a water bucket, but keep your eye on the knife with the ribbed black handle. When Billy starts shooting the bucket you'll be able to see that all three knives fall out and into the well (it is especially clear to see the black ribbed knife fall out). However, in the next scene the black ribbed knife is in the bucket again.