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3. The law if one transgressed and cooked a food of which there are opinions which permit it to be done even initially:[1]
(If a Jew himself[2]) heated a food of which there are opinions which permit this to be done even initially, such as to place a food that was completely cooked but has liquid which has completely cooled down, and [by placing it near the fire] it heated up there until it became Yad Soledes, then even though [transgressing such a prohibition] makes one liable to bring a Chatas offering (for the Jew) [if the Jew placed the food there], nevertheless, since there are opinions which allow this to be done even initially, [therefore based on the letter of the law] one may rely on their words after the fact (to not forbid the food).[3] [See Q&A]
The final custom with regards to one who reheated liquid which had fully cooled off:[4] [Despite the above] we are accustomed like the opinion which prohibits heating a liquid food on Shabbos, even in a case that one already heated it [and thus the food is forbidden to be eaten if this were done].
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[1] Admur 253:25; M”B 318:2
[2] Parentheses are in the original. See Ketzos Hashulchan 124 footnote 3 which says that Admur was in doubt in this ruling.
[3] Admur 253:25
[4] Admur 318:9