It is forbidden to eat or drink after sunset on Shabbat until once recites or hears Habdala. If, however, one had begun eating Se’uda Shelishit before sunset, then he may continue eating even afterward. Several Halachic authorities – including the Mishna Berura, Hacham Bension Abba Shaul, and Hacham Ovadia Yosef – maintain that if one did not begin Se’uda Selishit before sundown, he may begin afterward, as long as it is still within 13.5 of sundown. Thus, if a person was delayed for whatever reason, such as if there was a long line for Netilat Yadayim or Minha ended late, one may begin Se’uda Shelishit even after the sun has set, as long as 13.5 minutes have yet to pass since sundown. Preferably, however, one should begin eating Se’uda Shelishit before sunset.
If a person began eating before sundown, but he was not eating bread, then he must stop eating at sunset. This is the ruling of the Shulhan Aruch (Orah Haim 299:1). A person is allowed to continue eating past sunset only if he had eaten bread beforehand. There is, however, a debate among the Halachic authorities regarding the case of one who had begun eating a "Mezonot" food before sundown. Hacham Bension Abba Shaul (Israel, 1923-1998) ruled that "Mezonot" foods are treated like bread in this regard, and are considered a formal meal. As such, one who began eating a "Mezonot" food before sundown on Shabbat may continue eating even afterward. The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), however, ruled that "Mezonot" foods are not treated like bread, and thus eating such a food before sundown does not allow one to continue eating after sundown. Hacham Ovadia Yosef, in Hazon Ovadia (listen to audio recording for precise citation), takes a middle position. He writes that if one ate a "Mezonot" food before sundown, then he may continue eating after sundown, but only until nightfall, when the obligation of Habdala takes effect. This is contrast to other foods (besides bread), which one must discontinue eating once the sun sets.
Thus, one who began eating a "Mezonot" food before sundown should preferably stop eating once the sun sets, but if he wishes to continue eating, he may do so, until nightfall.
Once a person has recited Birkat Ha’mazon after Se’uda Shelishit, and the sun has already set, he may not eat or drink anymore until Habdala. Sometimes people want to suck a candy or have some nuts before Arbit on Mosa’eh Shabbat, but this is forbidden once a person has recited Birkat Ha’mazon if the sun has set, as discussed.
The Shulhan Aruch writes that water, as opposed to other beverages, may be drunk before Habdala on Mosa’eh Shabbat, even after sunset. However, the Arizal (Rav Yishak Luria of Safed, 1534-1572) advised against drinking even water before Habdala, noting that it could expose a person to danger. Hacham Ovadia thus writes that one is allowed to drink water before Habdala, but it is advisable not to do so.
Summary: Once the sun sets on Shabbat afternoon, one may not eat or drink until Habdala, unless he had begun eating Se’uda Shelishit with bread before sundown, in which case he may continue eating even afterward, until Birkat Ha’mazon. If one was delayed, and could not begin eating Se’uda Shelishit before sundown, he may begin after sundown, as long as he is still within 13.5 minutes of sundown. If one began eating a "Mezonot" food before sundown, he should preferably stop eating once the sun sets, but if he wishes, he may continue eating until dark. It is technically permissible to drink water before Habdala, though this should preferably be avoided.
