I have wondered about this for a while. It is my understanding that it is a mortal sin for a priest who is not in the state of grace to say Mass. Has it ever happened to anyone here that the priest on the way to Mass committed a mortal sin and because of that, had to cancel Mass because he couldn't go to confession? It has never happened at my chapel. I think it might happen because on the way to my chapel there are many immodest advertisements and many immodestly dressed women (especially in the summer time) who are all tempting the priests to sin.
When I was still going to the Novus Ordo, my family went to a weekday Mass about two to four times a month. Because we live in a rural area, the parish where our priest was during the week was some distance away so it was quite a drive.
On one occasion, and this only happened once, the priest said the Novus Ordo as usual, but when it came to communion, he pretended to receive. He clearly put the portion of host he had in his hand near his mouth and then covered it with his other hand and placed it back onto the paten. I happen to be looking directly at him at the time and I was immediately taken aback. Then I watched as he clearly did not actually drink from the chalice.
I wondered greatly at this, but I never discussed this with him. I do not know why he did not receive communion and I wondered what significance this had for what I thought at the time was the Mass.