Trimming and Piercing Errors. An imperfect piece misshapen because of being out of register when trimmed or pierced. For both trimming or piercing, a piece must rest in a nest for the work to be performed. Should a piece not be seated properly – off center or out of register – in the nest the trimming or piercing will be off center and not where intended. It is a trimming error when the outer shape is incorrect; it is a piercing error when the openwork is not where intended.