Leviticus 5:2-6

Or if someone touches any [ceremonially] unclean thing—whether the carcass of an unclean wild animal or the carcass of an unclean domestic animal or the carcass of unclean creeping things—even if he is unaware of it, he has become unclean, and he will be guilty. Or if he touches human uncleanness—whatever kind it may be—and he becomes unclean, but he is unaware of it, when he recognizes it, he will be guilty. Or if anyone swears [an oath] thoughtlessly or impulsively aloud that he will do either evil or good, in whatever manner a person may speak thoughtlessly or impulsively with an oath, but he is unaware of it, when he recognizes it, he will be guilty in one of these. [Mark 6:23] So it shall be when a person is guilty in one of these, that he shall confess the sin he has committed. He shall bring his guilt offering to the LORD for the sin which he has committed, a female from the flock, a lamb or a goat as a sin offering. So the priest shall make atonement on his behalf for his sin.