Gemara

Question: What kind of Mis'asek (someone engaged in a different activity) does "Asher Chata Bah" exclude?

If he was Mis'asek (and did Melachah) on Shabbos, a different source exempts this. The Torah forbids only "Meleches Machsheves" (intended labor)!

Shabbos 11b (Mishnah): One may not stand in Reshus ha'Yachid and (stick out his mouth and) drink in Reshus ha'Rabim, or vice-versa.

(Abaye): The same decree applies to Karmelis.

(Rava): The Isur of (transferring to or from a) Karmelis is only mid'Rabanan. Chachamim do not decree lest one come to transgress an Isur mid'Rabanan.

Support (for Abaye - Mishnah): A tailor may not go outside with his needle shortly before dark, lest he forget and carry it on Shabbos.

Suggestion: It is inserted in his garment. (It is not normal to carry this way. It is forbidden only mid'Rabanan.)

Rejection: No, it is in his hand.

Support (for Abaye - Beraisa): A tailor may not go outside with his needle inserted in his garment on Erev Shabbos close to dark.

Rejection: Beraisa #2 is R. Yehudah, who obligates a craftsman in such cases (but we do not decree due to mid'Rabanan laws).

(Beraisa - R. Meir): On Shabbos, a tailor may not go outside with his needle inserted in his garment, nor a carpenter with a ruler on his ear, If he did, he is exempt;

R. Yehudah says, a craftsman who carries something like people of his trade is liable (for it is normal for him to carry this way). Anyone else is exempt.

64b (Rav): Everything that Chachamim forbade to wear in Reshus ha'Rabim is forbidden in a Chatzer, except for Kavul (a cap worn under the headdress; alternatively, a signet of slavery) and a wig.

Bava Kama 26b (Rava): If a man did not know that there was a stone in his lap, and he stood up, and it fell (to a different Reshus, this is not a Torah Isur, for the Torah forbade only Meleches Machsheves.

Rishonim

The Rif and Rosh (Shabbos 5a and 1:27) bring the Mishnah verbatim.

Rambam (Hilchos Shabbos 19:21): A tailor may not go outside on Shabbos with his needle inserted in his garment, and a carpenter may not go outside on Shabbos with a piece of wood on his ear. If he went out, he is exempt. Even though he went out like people of his trade do, this is not the way of taking things out.

Rambam (26): A tailor may not go outside with his needle, nor a scribe with a pen, shortly before dark on Erev Shabbos, lest he forget and take it out (on Shabbos).

Tosfos (Shabbos 11a DH Shema): According to Rava, who does not decree Gezeirah li'Gezeirah, we do not forbid on Erev Shabbos lest one forget the needle. Even if he went out on Shabbos like this, it is not an Isur Torah, like it says in Bava Kama, for this is not Meleches Machsheves. Rather, we are concerned lest he forget to put away the needle before Shabbos, and on Shabbos he will remember, and forget that it is Shabbos.

Me'iri (11a DH Amar): If we are concerned lest he forget that it is Shabbos, he could take anything out of his house! Some say that seeing the needle in his hand make him forget that it is Shabbos. I say that really, we are concerned lest he remember that it is Shabbos, but forget the needle in his hand. Even though one is exempt for this on Shabbos, Gezeirah li'Gezeirah is only for something for which one who does it b'Mezid on Shabbos is exempt.

Poskim

Shulchan Aruch (OC 252:6): A tailor may not go outside with his needle in his hand shortly before dark, lest he forget and take it out (on Shabbos).

Beis Yosef (DH u'Mah): According to Rava, we do not decree lest someone transgress a mid'Rabanan law. Therefore, we forbid him to go out before dark with his needle in his hand, but if it is inserted in his garment, since on Shabbos itself it is forbidden only mid'Rabanan to go out like this, we do not decree to forbid it on Erev Shabbos. This is for other people. R. Yehudah and R. Meir argue about a tailor. R. Yehudah holds that if he goes out (b'Shogeg) when it is inserted in his garment, he is Chayav Chatas, therefore, we forbid shortly before Shabbos.

Beis Yosef (ibid., citing R. Yerucham): There is no difference between a tailor and other people. He forbids not only if it is needle in his hand, rather, even if it is inserted in his garment. It seems that the Rif agrees, for he simply cited the Mishnah verbatim. He did not bring Rava, who says that we do not decree for a decree. He holds that the Halachah does not follow Rava. Therefore, we forbid anyone, even if it is inserted in his garment. However, regarding standing in Reshus ha'Rabim and drinking in Reshus ha'Yachid, the Rif (Eruvin 32b) rules like Rava!

Rebuttal (Bach 11): The Rif in Eruvin merely brought the Mishnah, which can be like Abaye.

Beis Yosef (ibid.): We can ask this also against the Rosh, who wrote like the Rif in Shabbos and Eruvin! Perhaps they rule like R. Yehudah, who says that one who goes out when it is inserted in his garment is Chayav Chatas, therefore, we forbid shortly before Shabbos. The Mishnah is like R. Yehudah. The Mishnah mentions a tailor. It connotes that the Isur is only for a tailor. Therefore, they brought the Mishnah verbatim. They rule like Rava, who does not decree for a decree.

Rebuttal (Taz 8): I say that the Rif rules unlike Rava, because R. Yishmael permits wearing Tefilin shortly before dark because people do not forget Tefilin (but not because one is not liable for wearing Tefilin on Shabbos - Shabbos 12a, 61a). Also, the Gemara held that the simple reading of the Mishnah connotes that the needle is in his garment. Also, the Gemara could have asked whether the Isur to go out with a needle is even in Karmelis, or only in Reshus ha'Rabim, like it asked about drinking in another Reshus. It did not ask, for the Mishnah connotes that the needle is on his garment (i.e. surely we forbid Gezeirah li'Gezeirah). This is why the Rif rules like Abaye.

Gra (DH b'Yado): The Shulchan Aruch rules like Rava and R. Meir, for the Sugya is like R. Meir. However, it seems that the Halachah follows R. Yehudah, for we follow him against R. Meir, and the Stam Mishnah is like R. Yehudah.

Beis Yosef (DH Aval): The Rambam and Tur forbid a tailor to go out with his needle inserted in his garment. If he went out, he is exempt. They rule like R. Meir. They must hold that the Mishnah forbids all people. It mentioned a tailor only because this is a typical case. Since they exempt even a tailor if it was inserted, we do not forbid Erev Shabbos. The Mishnah forbids going out with it in his hand, since even on Shabbos he is not Chayav Chatas.

Magen Avraham (24): The only Chiyuv is if he intended for Melachah, but forgot that it is Shabbos or that this is forbidden, but not if he forgot that he is carrying them. This is like Tosfos and the Bach, unlike the Beis Yosef.

Bi'ur Halachah (DH Shema): The Me'iri answered Tosfos' question.

Gra (DH Aval): Rava agrees that we decree Gezeirah li'Gezeirah when it is likely that one will be negligent. Anything that one may not take to Reshus ha'Rabim, he may not take to a Chatzer, even if he is exempt in Reshus ha'Rabim (Shabbos 64a). This is because women are prone to take off ornaments, and a man puts down his Kelim. Chachamim did not decree shortly before dark, but they decreed just before dark. Therefore, one must check his pockets just when it becomes dark, even though he would be exempt if he went out on Shabbos (because he began walking before Shabbos).

Bi'ur Halachah (DH Samuch) The Pri Megadim and Nehar Shalom say that close to dark is a half-hour before Shki'ah. The Yerushalmi says that it is close to the time of Minchah, i.e. Minchah Ketanah (two and a half hours before the end of the day). The Poskim did not bring it, except for R. Chananel. In any case it is proper be stringent like the Pri Megadim and Nehar Shalom. It is clear from the Gra that 'close' is not just before Shki'ah.

Bi'ur Halachah (DH b'Yado): The Beis Yosef rules like R. Meir here and in Siman 301:12. The Gra rules like R. Yehudah, and so do the Me'iri and Bartenura. The Pri Megadim agrees that just before dark, all forbid even something for which one is exempt if he takes it out on Shabbos.

Magen Avraham (23): Nowadays we do not have Reshus ha'Rabim, so it is permitted before dark.

Mishnah Berurah (52): The same applies to something in his pocket, for it is normal to take things out in one's pocket. Since we do not decree Gezeirah li'Gezeirah, and nowadays we do not have Reshus ha'Rabim, one may carry something in his hand close to dark (before Shki'ah). However, many Poskim hold that nowadays we have Reshus ha'Rabim. According to the Gra, even Karmelis is forbidden close to Shki'ah.

Kaf ha'Chayim (77): The Mechaber (345:7) is among those who hold that we have Reshus ha'Rabim nowadays. (Note - he brings the opposing opinion as 'some say...' - PF.)

Mishnah Berurah (51): The same applies to other objects in one's hand.

Kaf ha'Chayim (75): The Poskim agree that we do not decree Gezeirah li'Gezeirah, therefore if it is inserted in his garment, it is permitted. However, the Bach says that ideally, one should not go out even if it is inserted in his garment. Sometimes one is liable even if it is inserted in his garment (301:8,12). In such cases, one should not go out close to dark even if it is inserted in his garment.

Kaf ha'Chayim (78): Some say that if it is wrapped in his hand, one is exempt for it on Shabbos. If so, it is permitted close to dark.

Mishnah Berurah (53): We are concerned lest he forget and after dark take the item from Reshus to Reshus, or carry it four Amos in Reshus ha'Rabim.