Doing a lesser prohibition to prevent a more severe one, the case of the cup of beer and the wrong bracha, calling off a hatzala call when there is no actual pikuach nefesh
Why the masechet opens with melechet hotzaa, is there is a chatzi shiur by this melacha, platas and returning cholent to the pot on shabbos, kreiva larayos of the Rambam and how its different than other prohibitions