Leviticus 5:1

Additional Sin Offering Regulations

5:1 “‘When a person sins in that he hears a public curse against one who fails to testify and he is a witness (he either saw or knew what had happened) and he does not make it known, then he will bear his punishment for iniquity.

Leviticus 5:18

5:18 and must bring a flawless ram from the flock, convertible into silver shekels, for a guilt offering to the priest. So the priest will make atonement on his behalf for his error which he committed (although he himself had not known it) and he will be forgiven.