Invalid sacraments within the Catholic Church
I wonder if it has been at all common in the ancient church that sacraments were performed invalidly within the Catholic Church. What did the church fathers think about it? Most church ordinances then and later considered the case of recognizing the baptisms of heretics and schismatics, but it was always assumed that the baptisms of the Catholic Church were valid. Nowadays, you may have heard about all kinds of variations on the matter and form of the sacraments, rendering them invalid.
I think the indefectibility of the Church implies that the approved forms and rites always constitute valid sacraments. Thus, claiming the invalidity of the post-Vatican II rites, for example, is inconsistent with regard to the indefectibility. On the other hand, this does not address the issue of unfaithful ministers.
I consider this issue disturbing, because during the Donatist controversy it was established that the Church has intrinsic holiness that no unfaithful minister can defile. However, by administering sacraments invalidly, they can, at least partly.