Shabbos Chapter 7, Mishnah 1. This Mishnah addresses the question of how many chatas (sin) offerings a person must bring for desecrating Shabbos, depending on which aspect of Shabbos he forgot. First a preface: a person brings a chatas for desecrating Shabbos only when he performed the labor unintentionally, without meaning to. Different things can slip a person's mind in the laws of Shabbos, and the Mishnah teaches how the obligation changes according to the type of forgetting.
"HaShocheiach Ikar Shabbos":
The Mishnah opens with a broad principle in the laws of Shabbos: a person who completely forgot the very concept of Shabbos - that is his unintentional error, that it slipped his mind that Shabbos exists in the world. Even though he performed many different labors over many Shabbosos, since the fundamental concept of Shabbos itself was lost to him, he is obligated to bring only a single chatas. One might have thought that many labors would require many chatas offerings, but the fact that the fundamental failure lies in forgetting the very existence of the mitzvah of Shabbos is what determines that there is only one obligation here.
One who knows the principle of Shabbos but forgot which day it falls on:
The next case: a person who knows that Shabbos exists, and performed many labors over many Shabbosos, except that he forgot when Shabbos falls and did not realize that this particular day was Shabbos. In other words, the concept of Shabbos is known to him, but he lost track of which day of the week it is, and this happened on several Shabbosos. In this case he is obligated to bring a chatas for each and every Shabbos, and not for each and every labor - even though he performed many labors on each Shabbos, he does not bring an offering for each one of them, but one offering for each Shabbos.
The Gemara explains that even if he does not remember the Shabbos in the middle, the very fact that a week has passed testifies to him that there was a Shabbos there. This is the meaning of the matter: Shabbos itself divides and separates one Shabbos from another. Once he knows that two weeks have passed, he knows that there were two Shabbosos there, and therefore he is obligated for each Shabbos on its own - but again, not for each labor on its own.
One who knows that it is Shabbos but does not know what the labors are:
The third case: a person who knew each time that the day was Shabbos, and performed many labors over many Shabbosos, except that he did not know which labors are forbidden on Shabbos - that was his mistake. In such a case he is obligated for each and every primary labor (av melachah) on its own. However, if he performed a single labor, such as planting, many times and in different ways, he is obligated to bring only one offering for the labor of planting, and it makes no difference how many times he did it or through how many subcategories and different forms of that same labor. It emerges that the maximum he can become obligated in is thirty-nine chatas offerings, corresponding to the count of the thirty-nine labors.
"HaOseh Melachos Harbei Mei'ein Melachah Achas":
And finally: a person who performed many different labors, but all of them are of the same category of one labor - he is obligated to bring only one chatas. Even if he performed the primary labor itself, such as planting, and also pruning and watering, all of which are subcategories of the labor of planting - he is obligated to bring only one chatas, since they are all forms of a single labor.
In summary: The Mishnah teaches four levels of obligation in chatas offerings for unintentional desecration of Shabbos: one who forgot the very principle of Shabbos - only one chatas; one who knows the principle of Shabbos but forgot which day it falls on - a chatas for each and every Shabbos; one who knows that today is Shabbos but does not know what the labors are - a chatas for each primary labor, up to thirty-nine chatas offerings; and one who performs many labors all of the same category of a single labor - only one chatas.