1) WHEN DOES BREAD BECOME BROWN
OPINIONS: The Mishnah (19b) says that one may bake bread in an oven on Friday only if its surface will become brown (crusted) while it is still day, before the arrival of Shabbos. Rebbi Eliezer requires that the bottom side of the bread become brown. The Gemara asks whether the "bottom side" refers to the part of the bread that is affixed to the side of the oven, or to the part that faces the fire underneath. The Gemara concludes that the part of the bread that is affixed to the side of the oven is considered the bottom of the bread.
What difference does it make whether the bottom is the side affixed to the oven, or the side facing the fire?
(a) RASHI, according to the simple understanding of his words, explains that the side of the bread facing the fire becomes brown quicker than the side affixed to the oven. The Gemara concludes that one may not bake bread on Friday unless there is enough time left for even the side that is affixed to the oven to become brown. The Rabanan argue and maintain that it suffices for one side to become brown -- that is, the side facing the fire (which browns first). The Rabanan are more lenient than Rebbi Eliezer.
(b) TOSFOS (DH Iba'i Lehu) does not accept Rashi's explanation. Tosfos bases his argument on the Yerushalmi that states that with regard to the Lechem ha'Panim, Rebbi Eliezer agrees with the Rabanan that a significant amount of the bread must become brown. This implies that with regard to Shabbos, Rebbi Eliezer maintains that only a small part needs to become brown (and Rebbi Eliezer is more lenient than the Rabanan). According to Rashi's explanation, the Yerushalmi is saying the opposite of what it should have said! It should have said that the Rabanan (who require, with regard to Shabbos, that only one side become brown -- the side facing the fire) agree with Rebbi Eliezer with regard to the Lechem ha'Panim (that a more significant amount of browning is required).
Therefore, the RIVA (cited by Tosfos) explains that the side facing the oven browns first. The Rabanan require that both sides become brown in order to be considered baked, while Rebbi Eliezer requires only one side to become brown before Shabbos. According to Rebbi Eliezer, it suffices if only the side facing the oven becomes brown.
(c) The PRI MEGADIM (in ROSH YOSEF) is bothered by the question that the Yerushalmi poses to Rashi's explanation. The Pri Megadim suggests that even according to Rashi, the opinion of the Rabanan is more stringent than the opinion of Rebbi Eliezer. The Rabanan require not only the side of the bread stuck to the oven and the side facing the fire to become brown, but all of the other sides also must become brown in order to permit this bread to continue baking on Shabbos. The Gemara's only doubt is in the opinion of Rebbi Eliezer, who argues with the Rabanan and is more lenient. The Gemara is unsure exactly how lenient he is. Does he require that only the side facing the fire be crusted, or that the side stuck to the oven be crusted as well (but not the other sides, which may crust later)?
We now understand the Yerushalmi's statement that Rebbi Eliezer agrees with the Rabanan that with regard to the Lechem ha'Panim we require a significant part of its surface to be brown. With regard to Shabbos, Rebbi Eliezer requires that only one (or two) side(s) be brown, while the Rabanan require all six sides to be brown. Regarding the Lechem ha'Panim, Rebbi Eliezer agrees with the Rabanan that all six sides must be brown.
2) BURNING REEDS OR SEEDS, SEPARATE AND BUNDLED
OPINIONS: The Gemara discusses the permissibility of lighting a fire of reeds or seeds before Shabbos that will remain lit on Shabbos. There is an argument in the Gemara whether -- in order to permit lighting the fire before Shabbos -- a majority of the reeds or seeds must be aflame before Shabbos or not. What is Rav Kahana's conclusion?
(a) The text of our Gemara (which is also the text of RASHI) differentiates between reeds and seeds. Reed-sticks burn better when they are not bundled together, and therefore when they are not bundled we do not require that a majority be aflame prior to Shabbos. Seeds, on the other hand, burn better when they are bundled together in a container, and therefore when they are not bundled we do require that a majority be aflame prior to Shabbos.
(b) According to the text of the ROSH, the Gemara concludes that according to Rav Kahana reeds and seeds are the same. When they are bundled together they burn better, and there is no requirement for a majority to be aflame.
(c) According to the text of the RIF, the Gemara concludes that according to Rav Kahana, both reeds and seeds burn better when they are not bundled together. Hence, when they are not bundled together there is no need for a majority to be aflame. When they are bundled together, a majority must be burning before Shabbos.
HALACHAH: The SHULCHAN ARUCH (OC 255:3) cites both the opinion of the Rif (c) and the opinion of the Rosh (b). The Shulchan Aruch apparently maintains that one should take into account both opinions and not light a fire with reeds or seeds prior to Shabbos unless a majority is aflame, whether or not they are bundled together.