Regarding a Chatas, it says "Asher Chata Bah (he sinned in it)" to exempt Mis'asek (one who did not intend to do the action).
Shabbos 72b: If one intended to pick up something detached, and picked up something attached, and detached it, he is exempt;
(Rava): If he intended to cut something detached, and he cut something attached, he is exempt;
(Abaye): He is liable.
Bava Kama 26b (Rava): If a man did not know that there was a stone in his lap, and he stood up, (if he inujured someone) he pays only Nezek (the loss of the victim's slave-market value). Regarding Shabbos (he is exempt, for), the Torah forbade only Meleches Machsheves (intended - Shemos 35:33).
Krisos 19b (Shmuel): One who was Mis'asek in Chalavim (or other food forbidden by Kares) or Arayos is liable, for he got pleasure.
Question: R. Eliezer cannot say that "Asher Chata Bah" excludes one who was Mis'asek in Melachah on Shabbos, for a different source exempts this. The Torah forbids only "Meleches Machsheves" (Which Mis'asek does it exempt?
Answer #1: According to Rava, we can say that he intended to cut something detached (from the ground, and cut it), but really, it was attached.
Answer #2: According to Abaye, we can say that he mistakenly thought that it is detached, and he intended to lift it, and detached it.
Question (Rav Yehudah - Mishnah - R. Yehudah): Even if he intended to pick dates and (mistakenly) picked grapes, or to pick black and he picked white, R. Eliezer is Mechayev Chatas, and R. Yehoshua exempts.
He is Mis'asek, yet R. Yehoshua exempts only for different Minim. He would obligate one who picked the same kind that he intended!
Answer (Shmuel): The case is, he originally intended to pick grapes, then forgot and thought that he wanted to pick dates, and when he tried to pick dates, he accidentally picked grapes. R. Eliezer obligates, for this was his original intent. R. Yehoshua exempts, for (at the time) he was not trying to do this.
Rambam (Hilchos Isurei Bi'ah 1:12): If one had Bi'ah with Ervah (a woman forbidden to him with Kares) k'Mis'asek, even though he did not intend, he is liable. The same applies to Chayvei Lavin and Sheniyos (Arayos mid'Rabanan).
Rebuttal (Ra'avad): What liability for a Korban applies (to Chayvei Lavin and Sheniyos)?!
Kesef Mishneh: The Magid Mishneh said that this must be a printing mistake in the Rambam. I say that the Rambam means that even though he is not lashed, he is liable to Shamayim like one who transgressed.
Rambam (Hilchos Ma'achalos Asuros 14:12): If one ate an Asur food in jest or Mis'asek, even if he did not intend to eat, since he benefited, he is liable like one who intended to eat.
Tosfos (Shabbos 72b DH Niskaven): Rashi says that Mis'asek is one who intended to pick up a knife, and cut Mechubar (something attached). Abaye and Rava argue about one who intended to cut Talush (something detached), and he cut Mechubar. All would obligate one who intended to cut Talush, and he cut different Talush. R. Tam challenged this, for in Krisos (19b), Shmuel says that one who was Mis'asek in Chalavim or Arayos is liable, for he got pleasure, but Mis'asek is exempt for Shabbos, for the Torah forbids only Meleches Machsheves. It is clear there that Shmuel exempts even one who intended to pick a fig, and accidentally picked a different fig. Rava (Shabbos 94b) exempts one who intended to throw four and threw eight, unless he said 'I will be satisfied wherever it lands', and all he more so if he intended to throw on one side and threw to the other side, for his intent was not fulfilled at all, or if he intended to cut this Mechubar and cut different Mechubar. Therefore, R. Tam says that here we discuss one who intended to cut something Talush, and cut it, but really, it was Mechubar. Abaye and Rava argue about what R. Eliezer exempts due to "Asher Chata Bah." Rava says that he exempts one who intended to cut Talush, and really, it was Mechubar. Abaye says that he exempts one who intended to lift it. Shmuel exempts due to Meleches Machsheves one who intended to cut Mechubar, and cut other Mechubar. His intent was not fulfilled. Later, the Gemara says that a case of Shogeg without intent in other Mitzvos is one who thought that that Chelev is Shumen (permitted fat) and ate it. This is just like Shabbos.
R. Akiva Eiger (Teshuvah 1:8): Rabah taught that if one did not know that there was a stone in his lap, and he stood up (and a Melachah resulted), regarding Shabbos, the Torah forbade only Meleches Machsheves. Tosfos (Krisos 19b) says that if he intended to cut something Talush, and really, it was Mechubar, this is called that he fulfilled his intent. However, "Asher Chata Bah" exempts this, for it is Mis'asek. The same applies to all Isurim. If one thought that an animal is Chulin, and he slaughtered it outside (the Mikdash), and he found that it was really a Korban, he is exempt. If he intended to cut Mechubar, and cut other Mechubar, only regarding Shabbos he is exempt, for his intent was not fulfilled. In other Isurim, e.g. he intended to slaughter a Korban outside, and he slaughtered a different Korban outside, he is liable. If so, one who did not know that there is a stone in his lap is exempt not only due to Meleches Machsheves. This is no worse than intent to cut something detached, and he cut Mechubar (which is exempt in all Isurim)! I say that Asher Chata Bah exempts from a Korban, but he did an Aveirah b'Shogeg. If something was not Meleches Machsheves, he did not sin at all. Rabah said that one who sat up pays Nezek and he is exempt for Shabbos. I.e. Kam Lei bid'Rabah Minei (one who is Chayav Misah is exempt from other punishments) does not apply, for he did not do a Melachah at all. If he were exempt only due to Mis'asek, he would be exempt from a Korban, but he did a Melachah, and he would be exempt from paying. The Poskim say that one who did not know that he has Chametz during Pesach did an Aveirah b'Shogeg. Chavas Da'as asked that he is Mis'asek, and he was forced to distinguish between Aveiros with and without an action. I say that Mis'asek transgressed, just he is exempt. If Ploni sees that David has Kilayim in his garment, but David is unaware, why must Ploni tell him, even if he is in public (Berachos 19b)? If the Torah does not forbid Mis'asek, Kavod ha'Briyos overrides mid'Rabanan laws! Rather, it is an Aveirah mid'Oraisa, just there is no Korban for it. However, one could say that Kilayim warms him, so this is like Chelev and Arayos, in which Mis'asek is liable. Tosfos (Shabbos 11a DH Shema) holds that Abaye forbids Mis'asek at least mid'Rabanan. We understand this if we say that it is like an Aveirah b'Shogeg. Even though on Shabbos it is not a Melachah due to Meleches Machsheves, mid'Rabanan it is forbidden. An application of this Chidush is if a slave says, on Shabbos, 'I will cut this Talush fruit' (and his master knows that it is Mechubar). One is commanded that his slave not do Melachah on Shabbos. If Mis'asek were not forbidden at all, he would not need to stop him. When a Tamei was unaware of the Mikdash, why is he liable? He thought that it is a Stam house. He is Mis'asek, like one who slaughtered outside what he thought that was Chulin! We must say that Asher Chata Bah exempts only from a fixed Chatas, but not from Oleh v'Yored, for which there is no verse. Especially according to what I wrote that Mis'asek is an Aveirah, just one is exempt from Chatas, we can say that he brings an Oleh v'Yored. This answers Tosfos' (19a) question against R. Tam. Helam Mikdash is totally exempt, for the verse discusses Helam Tum'ah, and Mis'asek exempts from Chatas. This is why in the Reisha, regarding one who ate Kodesh, R. Eliezer did not say that he is exempt for Helam Kodesh, like it says in the Seifa about the Mikdash. There, he totally exempts. Since the verse discusses only Helam Tum'ah, he brings Chatas Kavu'a for eating Kodesh, since there is no exemption of Mis'asek, since he benefits. He is totally exempt for Helam Mikdash due to Mis'asek.
Ohr Some'ach (Hilchos Gerushin 1:1): The Rambam says that if one had Bi'ah with Sheniyos k'Mis'asek, he is liable, i.e. to Shamayim (Kesef Mishneh), and he needs a Kaparah. Do not say that Mis'asek with Arayos is different because he benefited, for the Rambam said similarly about eating.
Note: Mis'asek with Chelev and Arayos is liable because he benefited! (Krisos 19a). The Radvaz and Kesef Mishneh (Hilchos Ma'achalos Asuros 14:12) say that the Rambam applies this to all forbidden food. It seems that the Ohr Some'ach disagrees.
Ohr Some'ach: This disproves R. Akiva Eiger, who said that Mis'asek transgressed and needs Kaparah. It also proves that one who transgressed a mid'Rabanan law b'Shogeg needs Kaparah. We can say that Mis'asek b'Chelev or Arayos, since he benefits, brings a Korban like one who was Shogeg. However, if Shogeg in a mid'Rabanan is not an Aveirah, Hana'ah is no reason to consider it like Mezid!