Gemara

(Mishnah): If any of the following were done by a Zar, or by one who was Tamei... he is exempt: Yetzikah (putting oil on a Minchah), kneading it...

Suggestion: The Mishnah did not mention Haktarah (burning). One who burned (b'Tum'ah) would be liable, i.e. Chayav Misah.

Answer: No, he transgresses only a Lav.

Inference: For Yetzikah, kneading..., there is not even a Lav!

Question (Beraisa): "Kedoshim Yihyu... v'Lo Yechalelu" forbids Yetzikah and kneading b'Tum'ah.

Answer: The Isur is mid'Rabanan. The verse is only an Asmachta.

Zevachim 80a (Mishnah - R. Eliezer): If blood that requires four Matanos (really, two Matanos on opposite edges, the blood goes on all four walls) was mixed with blood that requires one, we do four Matanos;

R. Yehoshua says, we do one Matanah.

R. Eliezer: You transgress Bal Tigra (detracting from a Mitzvah)!

R. Yehoshua: You transgress Bal Tosif (adding to a Mitzvah)!

Rosh Hashanah 28b (Beraisa): If a Kohen decided to add another Berachah to Birkas Kohanim, he transgresses Bal Tosif (even after the time for the Mitzvah passed)!

(Rava): One transgresses Bal Tosif at the time of the Mitzvah even without intent. One transgresses not at the time of the Mitzvah only with intent.

Rishonim

Rambam (Hilchos Mamrim 2:9): Since Beis Din can decree to permanently forbid something permitted, or temporarily permit an Isur, we must say that Bal Tosif and Lo Sigra is to add or detract and establish that this is Torah law. If Beis Din would permit cooking a Chayah with milk, this is Lo Sigra. If they would say that the Torah forbids fowl with milk, this is Bal Tosif. If they say 'the Torah permits, but we forbid', this is only a fence for Torah.

Rebuttal (Ra'avad): Anything Chachamim decreed for a fence is not Bal Tosif, even if they made it (seem) like Torah law and brought an Asmachta like we find in many places. Bal Tosif applies only to a Mitzvas Aseh.

Kesef Mishneh: The Rambam agrees that Bal Tosif does not apply to Chachamim's enactment. What is the Ra'avad's question? Also, why shouldn't Bal Tosif apply to Isurim, such as meat and milk?!

Sefer ha'Chinuch (454): Most say that Bal Tosif applies only to a Mitzvas Aseh.

Ma'ayan ha'Chachmah (cited in Hagahos in Minchas Chinuch): It says (Devarim 13:1) "Oso Sishmeru La'asos Lo Sosifu" to teach that Bal Tosif applies also to Lavin. (Shemor always refers to a Lav.)

Pri Megadim (Petichah Koleles to Shulchan Aruch, 1:35): In many places, a Beraisa expounds a verse, and Amora'im explain that it is only an Asmachta. The Ra'avad asks, how did the Tana make it seem like mid'Oraisa?! The Rambam holds that what the Great Sanhedrin enacts is mid'Oraisa (in most respects). Bal Tosif applies to an individual, or if the Great Sanhedrin says that their enactment is Torah law.

Ohr Some'ach: Sometimes, Amora'im never resolved whether a Drashah is mid'Oraisa or mid'Rabanan. How could the Tana'im transgress Bal Tosif?! I answer that what is expounded through the 13 Midos the Rambam calls 'mid'Rabanan', even though it is mid'Oraisa in every way. This is why another Beis Din can overturn it. The Rambam holds that Bal Tosif is only if they say that it is totally (i.e. explicitly) mid'Oraisa, e.g. that "Gedi" includes Chayos.

Minchas Chinuch (454 DH v'Hinei Af): The Rambam holds that there is no logic to distinguish an Aseh from a Lav. We do not totally permit anything for the sake of a fence. Sanhedrin 29a supports this. Surely, Adam told Chavah that they may not even touch the Etz ha'Da'as, and the Gemara calls this Bal Tosif. However, we do not learn Halachos from Agada.

Avnei Nezer (EH 8:11): We could say that R. Gershom's enactment not to marry two women was permanent. Bal Tosif does not apply, but the Cherem, which is mid'Oraisa, was temporary. If not, Bal Tosif would apply. This is why the Shulchan Aruch said that the enactment did not spread in all lands and the Cherem was only until the end of the fifth millenia, and the Rema added 'but the enactment is in all places.'

Chasdei David (Korbanos 8 DH va'Yomer p.133): The Rambam connotes that one transgresses only for a permanent addition to Torah law. However, many Gemaros proves that Bal Tosif applies even for adding once, e.g. Zerikah of blood that became mixed. The Rambam also says that it must be considered like Torah law. It seems that this is like R. Eliezer, who permits throwing extra blood as if it were water, for it is not done l'Shem Torah law. R. Yehoshua disagrees! It is Bal Tosif for a Kohen to add a Berachah on his own! We can answer the latter question like Rava said, that at the time of the Mitzvah, one transgresses even without intent (for Stam, he intends for the Mitzvah). The Torah commands twice about Bal Tosif. "Lo Sosifu" (Devarim 4:2) is in the plural. It forbids individuals to do an action that adds to a Mitzvas Aseh. If one adds to a Lav, e.g. he does not eat fat of the heart, this is not Bal Tosif on the Lav of Chelev. "Lo Sosif" (Devarim 13:1) is in the singular. It forbids Beis Din to transgress through words (a ruling). It applies even to Lavim. The Ra'avad holds that the Torah forbade Stam, i.e. in every case. Bal Tosif cannot apply to rulings of Beis Din, for Beis Din may make permanent enactments and decrees. The Ra'avad must explain (like the Gra below) that one verse forbids adding new Mitzvos, and the other forbids adding to exisiting Mitzvos. The Pesikta is like the Ra'avad, and Koheles Rabah (3:1(14)) is like the Rambam.

Gra (Aderes Eliyahu Re'eh 13:1 (p.101:1)): "Lo Sosifu" (Devarim 4:2) refers to adding new Mitzvos. "Lo Sosif" (Devarim 13:1) refers to adding to exisiting Mitzvos, e.g. extra Zerikos of blood.

Ramban (Devarim 4:2): Also making a new Mitzvah is forbidden. Chachamim were debating whether we may be Mechadesh Mikra Megilah, until they found verses for it. The Torah commands Chachamim to make fences, as long as it is known that it is a fence.

Birkei Yosef (Sof Siman 243): The Ramban questioned why Rashi cited Beraisos that imply that the Torah forbids Amira l'Nochri (telling a Nochri to do Melachah on Shabbos). The Mizrachi answered that if people think that it is mid'Oraisa, this is even better (they will be more careful not to do it)! Some ask that according to the Rambam, this is Bal Tosif! The Ramach answered that the Rambam said so only about matters like meat of Chayos with milk, lest one think that there are lashes mid'Oraisa for this. We are not concerned for Amira l'Nochri, for it has no action (and hence no lashes).

Rashba (Rosh Hashanah 16a DH Lamah): Bal Tosif is what a person decrees on his own. An enactment of Chachamim is not Bal Tosif.

Minchas Chinuch (454 DH v'Hinei Dinim): Chachamim are authorized to make enactments of Mitzvos Aseh such as Netilas Yadayim and Ner Chanukah, and also Lavim. Even an individual may establish an annual celebration of a miracle done for him. Even if Chachamim say that something is mid'Oraisa, this is not Bal Tosif (unless they bring a verse for it), for anyone can see that it is not in the Torah.

Poskim

Shulchan Aruch (OC 31:2): One may not wear Tefilin on Chol ha'Mo'ed, for the day itself is an Os (sign of Yisrael's allegiance to Hash-m. Wearing Tefilin is a disgrace to the Os of the day.)

Rema: Some say that one must wear Tefilin on Chol ha'Mo'ed. This is the custom in Chutz la'Aretz. One blesses on them, but not aloud in the Beis ha'Keneses.

Taz (2): One may wear them without a Berachah. The Zohar discusses a great punishment for one who wears them. The Rosh concluded that one may bless; he did not see the Zohar. Also Hagahos Maimoniyos says to wear them without a Berachah; Bal Tosif is only with intent to add.

Magen Avraham (2): He does not transgress Bal Tosif because he wears them only due to the Safek that he is obligated.

Eshel Avraham (2): He should stipulate 'if I am exempt, this is not l'Shem Mitzvah.'

Bi'ur Halachah (DH Hayah): If he does not intend l'Shem Mitzvah, one may wear Tefilin even on Shabbos and Yom Tov.