I have written before about the frantic desire some who think of themselves as Ultra-Traditionalists have to discover 'invalidity' in the sacraments of the non-traddy part of the Church. Indeed, there are ultra-ultra-traddies who passionately seek out 'invalidity' even among plain traddies.
The last time I wrote about this, I quoted the locus classicus among traditional Catholic theologians discussing 'validity' - a passage of S Robert Bellarmine. I don't feel like repeating either the passage or my detailed explanation: you will find them both by tapping Bellarmine into the box at the top left-hand corner of the page. Here, today, is an explanation by an Englishman, the Fr Adrian Fortescue who wrote The Ceremonies of the Roman Rite Described, still the standard handbook for the Vetus Ordo . This is what he wrote in a book published by the Catholic Truth Society in 1908.
"People who are not theologians never seem to understand how little intention is wanted for a sacrament (the point applies equally to minister and subject). The 'implicit intention of doing what Christ instituted' means so small and vague a thing that one can hardly help having it ... numbers of Catholics confused intention with faith. Faith is not wanted. It is heresy to say that it is (this was the error of St Cyprian and Firmalian against which Pope Stephen I, a.d. 254-7, protested). A man may have utterly wrong, heretical and blasphemous views about a sacrament, and yet confer it or receive it validly."
His powerful point is that this is an area in which one can fall into heresy. If you say that "Fr X does not have one ounce of Catholic belief about the Mass, so his Masses are obviously invalid, because he doesn't intend to do what the Church does", you are not being a rigidly firm splendidly orthodox Traditional Catholic, defending with your lifeblood the Faith of our Fathers. You are being a heretic. Well, no, not really: because you don't want to contradict the Church's established dogma about 'intention'; you are just terribly confused about what that teaching is. But what you have actually said still technically is, in Fortescue's word, 'heresy'. Seriously: the danger for us all at this time is that some of those trying to restore Tradition will erect new and untraditional home-made shibboleths of their own. This is not a help; it is an unwelcome distraction from the real problems.
I occasionally get the impression that some ultra-traddies desperately need untraddy sacraments to be invalid because they see this as essential to validate an exclusivist stance. I rather suspect that those who do this are not only lapsing into error; they are guilty of a want of charity in desiring their fellows not to be fed with true sacraments. What they would do better is to attack extreme antitraddy clergy of sacrilege: of defiling by abuse sacraments which are valid (if sacraments are not valid, then sacrilege does not exist).
The only way a daft priest can invalidate a Mass is by forming a deliberate interior intention "I'm not performing any sort of Christian rite at all; I am just play-acting"; or by not using real wheat bread and real grape wine; or by not using words which mean This is my Body, This is my Blood. There are a lot of other bad things he can do, but they are what we call "abuses" and abuses do not invalidate. They might, however, give one a thoroughly good reason for seeking out a Mass celebrated by a less daft priest.