The Gemara in Shabbos says that Bibi bar Abaye asks if one can violate a Rabbinic commandment (unsticking a loaf from an oven) to save oneself from being "chayav a chattas".
The Gemara says that there can be no instance when one will be chayav a chattas (since one would have to be a shogeg both when one stuck the bread into the oven and when it baked. If he's taking it off, he knows what's going on and isn't a shogeg).
Therefore, the gemara says the question was whether one can do it to avoid an Issur Skila.
Why not ask it the original way (Safek into the oven and Vadai when it baked) and there is no chiuv, but the same Issur is still there? Why do we care if one has to actually bring a sacrifice?
