Jamieson, Fausset and Brown on Leviticus 5:17-19

17-19. if a soul sin ... though he wist it not, yet is he guilty--This also refers to holy things, and it differs from the preceding in being one of the doubtful cases,--that is, where conscience suspects, though the understanding be in doubt whether criminality or sin has been committed. The Jewish rabbis give, as an example, the case of a person who, knowing that "the fat of the inwards" is not to be eaten, religiously abstained from the use of it; but should a dish happen to have been at table in which he had reason to suspect some portion of that meat was intermingled, and he had, inadvertently, partaken of that unlawful viand, he was bound to bring a ram as a trespass offering [Le 5:16]. These provisions were all designed to impress the conscience with the sense of responsibility to God and keep alive on the hearts of the people a salutary fear of doing any secret wrong.