The Minchas Chinuch presents a radical chiddush in this week’s parsha. The halacha is that if someone is dangerously ill and issurim must be violated to help that person, it is better to minimize the extent of the issurim violated – ma’achilin lo hakal hakal techila, we feed the ill person the lighter issur first. The Rishonim debate how one measures which actions entail greater or lesser issurim – is it the number of issurim violated or the severity of the issur? For the sake of his argument the Minchas Chinuch presents the Ran’s view that the number of issurim violated is the determining criteria. If so, given that in our parsha there is an issur of ‘lo tasur’ for violating the words of Chazal and a mitzvas aseh to listen to the words of Chazal, one might very well have a situation where a law derived from a derasha carries more weight than a law which is explicit in a pasuk. Minchas Chinuch’s example: if one eats a non-kosher bird in violation of explicit pesukim in the Torah, one violates a lav and and an aseh (the mitzvas aseh of eating only kosher food). The gemara learns from a derasha that that there is a lav and mitzvas aseh not to eat the bird which was shechted as part of the ceremony of being metaheir a metzora. Since these two issurim are derived from a derasha, violating these issurim would also be a violation of the mitzvas aseh of listening to Chazal and the lav of disobeying their words. So 4 issurim to 2, it is better to feed a sick person a non-kosher bird than to feed a sick person a kosher bird shechted properly which was used for taharas metzora!
There are a few reasons one might argue on this chiddush, but I won’t spoil the fun of letting you mull it over. R’ Elchanan discusses in Kuntres Divrei Sofrim 1:35-39.