If one sharpens a knife on Shabbos, have they transgressed an issur d'oraisa or an issur d'rabbanan?
The answer depends on the nature of the knife and how sharp it was to start with. If it is a knife that can only be used once it is sufficiently sharp and you sharpen it, the issur is de'oraita and it falls into the category of makkah bepatish. If it is a knife that does not require sharpening and you have merely done something with it that inadvertently caused it to be sharper, that is only a problem - and only according to some authorities - if doing so is the sort of thing that you would normally do during the week. If it is, you have violated the rabbinic prohibition of shevut. If it is not, then you have not done anything.