Leviticus 5:2

Or if a man touch anything, unclean, whether the carcase of an unclean beast, or a carcase of unclean cattle, or the carcase of an unclean reptile, and it be hidden from him, and he, being unclean, shall touch any consecrated thing, he is guilty.

or anyone who touches any ritually impure thing This denotes the carcass of one of the four enumerated beasts (the swine and its fellows) [11:4–7].

or the carcass of a ritually impure beast i.e., ritually impure as food, like the horse or the donkey a ritually impure creeping being one of the eight enumerated [11:29–30]

he was unaware of it and afterward found out. This applies also to someone who touches a man who suffers from a constant flow, someone who touches someone else who had contact with a corpse, someone who touches a woman who suffers from a constant flow, and someone who touches a menstruant.

AND IT IS HIDDEN FROM HIM — “that is, he had forgotten his state of uncleanness. AND HE INCURRED GUILT — by eating holy food or by entering the Sanctuary.” This is Rashi’s language. Now [Rashi’s] intent is not that the [requirement of] eating the holy food or entering the Sanctuary is derived from the word v’asheim (and he incurred guilt), for this word is used with reference to all the sin-offerings mentioned previously. But this section deals in a brief manner with those points which are self-understood. For there is no sin involved in touching a carcass of an unclean beast or swarming thing [which defiles the person]. Even the priests have not been warned against it. So it is impossible that Scripture should require a person to bring an offering just because he touched them and thereby became defiled. Rather, Scripture is stating that when a person becomes defiled and forgets his state of uncleanness, or when he swears [an “oath of utterance”] and forgets the oath, and he incurs guilt by violating it — for either of these sins committed through forgetfulness, he is required to bring an offering. Now it is self-understood that the mere forgetfulness of his state of uncleanness involves no sin, except [if in that state] he ate holy food or entered the Sanctuary. Nor is there any sin in the mere forgetfulness of an oath, except if he violated it. This is the literal explanation of the verses in this section. Our Rabbis have further interpretations on this section strengthening this explanation.

ונעלם ממנו והוא טמא אשם, he had forgotten that he was ritually impure and entered the holy domain, or he had eaten sacred meat while being in such a state of impurity, offences punishable by karet if perpetrated knowingly.