A loony priest.
In the Catholic Church, the service books are there for a purpose. Consider the history of Bishop Arthur Roche: it turns out that, after all he was neither a retired figure-skater nor a cinnabon-addict (these were little stories invented by an obscure journalist called Damian Thompson). (In this spirit of honesty I must also admit that the claim that my Auntie Moly drank a bottle of Damian's hair-restorer was also slightly exaggerated.)
Back to Uncle Arthur. One good thing he did was to produce a new and accurate translation of the Missal, to replace the 1960s free-format fantasy on a theme that was scribbled down in a restaurant in the aftermath of Vatican II. The new translation annoys the Tablet, so it must be good. Now, if someone has bothered to provide a translation, and print it in a book, and if the bishops tell you to use it, you should at least stick to the text, Father.
+Arthur also got a reputation for closing churches. Not a Damian Thompson invention.
At three times in the service the priest is supposed to say "The Lord be with you." This wasn't good enough for Father Calamity, who insisted on saying "The Lord is with you." This is cheating, a little, as it invites the reply "And with your spirit". But he saved the worst until the end:
"The Mass is ended. Go in Peace." (Not quite right, but not too bad.)
"Thanks be to God."
"And thank you too." (WHAT???)
A natural reaction at this stage.
FATHER E replies:
Well, there are several possible courses of action here:
(1) Walk past him as he stands outside at the end of the service, and say nothing apart from a polite "Good morning".
(2) Talk to him about his eccentric behaviour, politely pointing out that he should stick to the printed words.**
(3) Potter around after the service, lighting votive candles, until the priest has gone.
**In Anglican churches, remoulding the liturgy in your own image may be perfectly OK, of course. Still, I do know a Baptist who, after attending an Anglican service, cornered Rowan Williams and taxed him with some alleged theological error in his sermon. Good luck with that one.
You will probably find that (3) is the least likely to endanger your soul. If Fr M is still away next week, you may wish to try a different church. Of course, this is sometimes considered to be provocative behaviour, and may lead to Fr Catastrophe's leaving the priesthood altogether and buying a rainbow cat, but you are not really to blame here.
The sort of cat that may suit a wayward priest.