Now we will discuss cases in which a person may be obligated to bring multiple chatas offerings.
One lapse of awareness with one prohibition:
If a person ate chelev - the forbidden fat, for which one who eats it inadvertently must bring a chatas - and ate an olive's volume and then another olive's volume. If both acts of eating occurred during "one lapse of awareness", meaning that from the first act of eating until the second he remained completely unaware that the matter was forbidden, it makes no difference whether an hour passed between acts of eating or twenty-five years: he brings only a single chatas for the entire duration of the lapse of awareness in which he ate chelev without knowing it was forbidden.
Consequently, if a person ate chelev for breakfast every day for twenty-five years, even though he did so a thousand times, since he did not know it was forbidden, he brings only one chatas - at the end of the period, once he becomes aware that his actions were forbidden.
One lapse of awareness with four types of prohibition:
Conversely, if in "one lapse of awareness" he ate food containing four forbidden ingredients, each of which is a separate Torah prohibition - such as one who thinks the sandwich before him is completely kosher, but in fact it contains four types of severe prohibitions for which one incurs kares - he must bring a separate chatas for each of the four ingredients, provided he ate an olive's volume of each.
These are the four things:
Chelev - the forbidden fat.
Dam - blood, literally.
Nosar - sacrificial meat left over after its allotted eating time.
Pigul - sacrificial meat during whose offering an improper thought was held regarding the time at which any part of the sacrificial service would be performed.
Therefore, one who eats four olive's volumes, an olive's volume of each of these four things, brings a chatas for each of the four transgressions separately, since they are different mitzvos out of the 613 mitzvos, even though it was done in one lapse of awareness.
The Mishnah is saying: In this respect, there is a stringency with different types of prohibitions from the 613 mitzvos that does not exist with a single prohibition. One who eats four chelev sandwiches on four days during one lapse of awareness brings only one chatas, whereas one who eats a single sandwich containing chelev, nosar, pigul, and blood brings four chatas offerings for a single act. Thus, with multiple types, the law is more stringent.
Conversely, a stringency with one type that does not exist with multiple types:
If he ate half the requisite measure, half an olive's volume of chelev, and then ate another half measure: "Mimin echad" - from one type, meaning both halves are of the exact same prohibition, chelev and more chelev - they combine to form the complete measure of an olive's volume, "vechayav" - and he is obligated to bring a chatas offering. But "mishnei minin - patur" - from two types, he is exempt: if he ate half an olive's volume of chelev, half an olive's volume of nosar, and half an olive's volume of pigul, there is no obligation to bring a chatas. This does not mean there is no prohibition in such eating - it is all forbidden by Torah law - but rather that he did not complete the requisite measure of an olive's volume in any single category of prohibition, and therefore there is no chatas obligation.
The Two Dimensions of Halachic Eating:
Every halachic act of eating - whether eating matzah on Pesach or any other eating - has two dimensions: the minimum required measure, which is a kezayis, the volume of an olive, and the time frame within which this measure must be eaten. When it is stated that one who eats half a kezayis of chelev and another half a kezayis is liable for a chatas, it means that he ate the two halves within a short period of time, in such a way that they combine to form a complete kezayis measure.
In summary: We learned that one who eats a single type of forbidden food in one lapse of awareness, even many times and over the course of years, brings only one chatas; whereas one who eats a kezayis of each of four types of forbidden foods - chelev, blood, nosar, and pigul - in a single lapse of awareness is liable for a chatas for each and every one, since these are distinct mitzvos of the 613 mitzvos. On the other hand, half-measures combine for chatas liability specifically when they are of one type of prohibition, and not from two different types.
And how long is that time frame within which half-measures combine? This is the subject of the next Mishnah.