ולקח למטהר שתי צפורים חיות טהרות... ושחט את הציפור האחת... ושלח את הצפור החיה על פני השדה
As part of the process of purifying the metzora, two birds are brought as an offering to Hashem. Only one of them is killed, however, while the other one is set free. Birds are used for the korban, sacrifice, because birds are constantly chirping, and the metzora had not stopped spewing his slander when he should have kept his mouth shut. This explains why one bird is killed, but what about the second bird? Why is it released? The explanation as cited by the commentators is that the Torah is hereby circumventing a common error, thus imparting an important lesson….