When the mitzvah and the prohibition essentially come together, as with shaatnez and tzitzit - such that the positive commandment overrides the prohibition. Where else does this happen? For example, the metzora and the prohibition against shaving pe'ot. What about a kohen who's a metzora? With all concomitant objections. Of course, it is also relevant and present in the context of yibum and the prohibition of "eshet ach," the wife of one's brother. Also, the question of whether a positive commandment can override a negative commandment that has a punishment of karet? For example, brit milah on Shabbat. The Korban Pesach. The Korban Tamid. All in all, the power of deriving halakhah from the biblical text.