A shvus dshvus (rabbinic prohibition on top of a rabbinic prohibition) is allowed במקום צורך (in a case of need). But what is considered במקום צורך? And is a shvus dshvus allowed for anything besides במקום צורך and the performance of a mitzvah? If so what?
EDIT-for those who want to know what a shvus dshvus is, the Wikipedia link רבות מחשבות gave has a pretty good explanation. As for how I know it's allowed for mitzvah, sick people, and great need, the Wikipedia page says that the Rambam says it (Hilchos Shabbos 6:9). Additionally it mentions it in the 39 Melochos book by asking a non-Jew to do a drabbanun (rabbinic prohibition). And I've seen it a lot in the Gemara (like Tosfis explaining why Rebbi Yehuda would allow first sex on Shabbos since it's destructive. It's somewhere in Kesubos (5b s.v. "Im").