Open memo to guest celebrant:
If you make up your own words, you might distract the organist. He will forget to play the music for the Agnus Dei. The cantor will lean over and remind him...but he is deaf in one ear, and he will not understand. She will have to tell him in a louder voice. The organist will become flustered, stumble onto the bench, and begin to play in an odd key. The cantor, in her turn, will be distracted, and sing the wrong line...the choir will hold its collective breaths. Then, suddenly, everyone will fall back into the normal setting, and the Mass goes on as it should.
God bless your next mission, Father!