"If a person sins and commits one of the commandments of G‑d which may not be committed, but he does not know, he is guilty, and he shall bear his transgression. He shall bring an unblemished ram from the flock, with the value for a guilt offering, to the kohen. The kohen shall then make atonement for his unintentional sin which he committed and did not know."—Leviticus 5:17-18.
One who is uncertain whether he committed a sin whose transgression would require him to bring a Sin Offering – e.g., two pieces of fat were before him, one permitted and one forbidden; he consumed one of them and the other was then lost (so that it was impossible to identify whether it was of the forbidden or permitted variety) – he must then bring an Asham Taluy (a "Doubtful Guilt Offering").
(If afterwards it was conclusively ascertained that he ate of the forbidden fats, he must bring a standard Sin Offering [even if he has already brought his Asham].)