Concerning that which was already asked here: https://dinonline.org/2012/07/02/rabbinic-punishment-of-kareis/
In regards to Koreis M’drabbanan, and based on the response there — What about in a case of a chatzitzah by tevillah of a Niddah, for something that is only chotzetz m’drabbanan. If the isha is meshemash before returning to the mikveh, could this be called “Korais M’drabbanan,” or do we simply say she has transgressed a rabbinic prohibition like any other? Perhaps a nafka minah might be l’inyan teshuva (if any…)?
An interesting case is a woman who didn’t wash her hair (chafifah) before tevilah.
The Shulchan Aruch (based on a number of rishonim, including the Rashba in the name of the Raavad, the Ramban, and the Meiri) writes that if a woman did not do chafifah, it is considered as though she did not do tevilah — even if her hair was found to be without knots, clean, and so on.
This is of course a rabbinic law, and the statement whereby it is “as if she did not do tevilah” can perhaps be a case of “kareis derabanan.”