Here's a theoretical: if someone has to save a life and has two equally easy options, both of which violate halacha, one d'oraita and one d'rabbanan. Should he choose the d'oraita or the d'rabbanan?
At least with regards to hilchos Shabbos, you should choose the derabannan.
See שמירת שבת כהלכתה לב:כז-כח*, who writes that if there is no difference in speed or quality of care, that one should choose to do a rabbinically prohibited action, instead of one that is Biblically prohibited.
I don't know if this is Shabbos-specific, or if this rule applies universally.
* Second edition, red cover