Drinking liquids that were stored in metal containers:
One is not to drink beverages mixed with water that have stayed overnight in a metal container or a container made of Neser. (It goes without saying that plain water which stayed overnight in a metal container [is forbidden to drink].) [However, beverages that do not contain water that stayed overnight in a metal container may be drunk. Furthermore, some Poskim rule that the prohibition is only with watered down beverages and not plain water, and plain water may be drunk even if it stayed overnight in a metal container. The Rebbe Maharash however was stringent regarding even plain water, as were Rabbanei Anash of his generation, and he would thus not leave hot water for the Shabbos tea overnight in a metal urn. Practically, the custom today is to no longer be careful in this matter at all, even regarding watered down beverages, and certainly regarding plain water. One may even initially rely on this custom even if he is G-d fearing and a Torah scholar, and is generally particular regarding such matters. There is thus no need to abstain from drinking canned soda or other canned foods with liquid. Likewise, one may leave water overnight in a kettle of electric urn, such as for the sake of Shabbos.]
The custom is to no longer be careful against leaving liquids overnight in metal containers and there is thus no need to abstain from drinking canned soda or other canned foods with liquid. Likewise, one may leave water overnight in a kettle of electric urn, such as for the sake of Shabbos. Even from Talmudic law, this prohibition only applies to water-based liquids. The following Q&A deal with the Talmudic prohibition:
If the liquid contained a food in it overnight, does the Talmudic prohibition apply?
May the liquid that stayed overnight be used to mix into foods according to the Talmudic prohibition?
Perhaps even by liquid that stayed overnight the prohibition is only against drinking the liquid, while it remains permitted to mix it into food.
If the liquid remained in a metal container that is attached to the ground overnight, does the Talmudic prohibition apply?
The prohibition only applies to liquid which remained in a metal vessel that is detached from the ground, while if it is attached to the ground then the liquid is permitted. [Thus, the prohibition does not apply to water that stayed in pipes overnight.]
One Shabbos many guests came to Lubavitch, including many Rabbanim of Anash. On Shabbos morning, Rebbitzin Rivka, the wife of the Rebbe Maharash, offered the Rabbinic guests some tea. When they sat on the table, the worker placed a large copper urn on the table that held the hot water and poured them a cup for them to make themselves tea. The Rabbanim began discussing the Halachic issue with the water, due to the above prohibition mentioned in the Talmud and extended by Admur to apply even to plain water. Nonetheless, they felt obliged to come up with a Halachic explanation to justify its allowance, as if the Rebbe Maharash is drinking it then certainly it must be allowed. Each Rav gave his own justification. One of the Rabbanim suggested that it is permitted out of Kavod Shabbos, which is a Mitzvah “Ki Shomer Mitzvah Lo Yada.” Another suggested that since it was insulated in the oven, and the oven is attached to the ground, therefore the urn loses its status as a vessel and the prohibition does not apply. The Rebbe Maharash suddenly entered the room holding the exciting discussion of the Rabbanim and settled the matter once and for all. The Rebbe Maharash asked the worker to bring the earthenware urn to the table to show the guests that the water stayed overnight in the earthenware urn and was simply poured now into the copper urn in order not to place an earthenware vessel on the table.
 Admur C.M. Hilchos Shemiras Guf Vehanefesh Halacha 7; Rashbi in Nida 17a; Yerushalmi Brachos 7/5; Rosh Brachos 8/2
 Lit. “Mashkin Mezugin”; Admur ibid; Yerushalmi ibid; However, see Bneiy Tziyon 1/10-11 that Mashkin Shnimzagu refers to liquids that were poured for drinking purposes. The Rebbe in Igros Kodesh 2/140 and 144 negates this explanation
 What is Neser? Neser can be one of many compounds: 1) Sodium Hydroxide [See Yerushalmi Shabbos 9/5; Tiferes Yisrael Hakdama Taharos 44]; 2) Alum [See Avoda Zara 33b and Rashi there; Rash; Rosh Keilim 2a; Meiri Shabbos 16; Radak Yirmiya 2/22]; 3) Chalk [Rambam Keilim]. See Aruch Erech Neser; Sefer Hashorashim p. 330; Tashbatz 1/28. The common denominator of all these vessels is that they are no longer commonly in use.
 The reason: One who does so “brings blood upon his head and is liable for his life” [Rashbi in Nida ibid] as the Ruach Rah resides on this liquid. [Zivcheiy Tzedek 116/61; Kaf Hachaim Y.D. 116/92]
 Admur ibid in parentheses; brought in: Over Oreiach in Orchos Chaim 4/2; Maharsham 2 Mafteichos Y.D. 8; Tehila Ledavid 4/8; see Admur O.C. 455/19; Bneiy Tziyon 1/10; 2/3; Igros Kodesh 2/140-145 and that Admur was in doubt regarding this ruling and thus placed it in parentheses; Machshavos Eitza 5; Divrei Yatziv 33/2; See other opinions below and coming footnotes
Contradiction in Admur: Admur O.C. 455/19 rules regarding Mayim Shelanu that remained in a copper vessel eve even for many days it may be used. This contradicts the ruling of Admur ibid. The following answers can be offered: 1) Admur in 455/19 refers to water that stayed in a copper container during the daytime and not overnight; b) It is permitted to mix the liquid into foods, and the prohibition is only against drinking it. [Igros Kodesh 2/145] c) The laws of Shemiras Haguf were written much later than the laws of Pesach which was the first Halachic work written by Admur, and Admur only innovated this ruling that the prohibition applies even to plain water in his later writing. This also explains why it was written in parentheses, as it is a personal novelty of Admur not brought in previous Poskim. [Shulchan Melachim 2/51 footnote 3; See Igros Kodesh 2/140]
 Implication of Admur ibid; However, see Bneiy Tziyon 1/10-11 that Mashkin Shnimzagu refers to liquids that were poured for drinking purposes. The Rebbe in Igros Kodesh 2/140 and 144 negates this explanation
 Implication of Admur O.C. 455/19 “However, Bedieved if the Mayim Shelanu remained in the copper vessel even for many days it may be used”; [see previous footnotes for explanation or contradiction]; Olas Shabbos end of 456; Chok Yaakov 455/10; Peri Chadash 455/1; Over Oreiach in Orchos Chaim 4/2 questions the ruling of Admur in C.M. ibid
 Retold by Rav Zalman Shimon Dworkon in Shmuos Usippurim 2/49-50; See Maaseh Shehaya brought in end of this Halacha
 Custom of Jerusalem regarding plain water, brought in Igros Kodesh ibid
The reason: As when the public becomes accustomed to perform a danger Chazal state “Shomer Pesaim Hashem.” [Shabbos 129b; See Tzemach Tzedek E.H. 11] Furthermore, when the danger is associated with matters of Segula [unnatural] then only when people beware from doing it does the Segula danger apply, while if people are no longer careful then the danger subsides completely. [Igros Kodesh 2/144] Alternatively, today the Ruach Raah no longer resides and only applies in Talmudic times, just as we rule regarding Zugos. [Rashal Yam Shel Shlomo Kol Habasar 12; See regarding garlic: Zivcheiy Tzedek 116/61; Kaf Hachaim Y.D. 116/92]
 Igros Kodesh 2/144
The reason: As when the danger is associated with matters of Segula [unnatural] then only when people beware from doing it does the Segula danger apply, while if people are no longer careful then the danger subsides completely. [Igros Kodesh 2/144]
 Suggestion made by questioner in Igros Kodesh 2/144; So rule regarding garlic: Zivcheiy Tzedek 116/61; Kaf Hachaim Y.D. 116/92
 Igros Kodesh 2/144
 Igros Kodesh 2/145 in explanation of Admur 455/19
 Igros Kodesh 2/144
 The reason: As an item that is attached to the ground loses its status as a vessel. [ibid]
 Retold by Rav Zalman Shimon Dworkon in Shmuos Usippurim 2/49-50