Bon jour bon moys
by Guillaume Du Fay
No Ficta
The problem with this passage is that the flat in the signature creates a melodic tritone between Bb and E-natural in the bottom voice (the tenor) in measure 4.
Besseler Version
Heinrich Bessler’s solution in the Dufay Opera Omnia edition is to raise the Bb in the tenor (the bottom voice), then, in imitative fashion, in the cantus part as well. This removes the melodic tritone in the tenor but creates pseudo-cadential motion in the middle of the phrase (of the text phrase as well, even though the text is not given here) and at a place that is not strongly indicated by the counterpoint.
Alexander Blachly of Pomerium takes this even further and raises the cantus Bb in m. 5 as well, as can be heard in the Pomerium recording of this piece.
Duffin Version
My own solution, from my Forty-Five Dufay Chansons edition, is to apply the “una nota” rule to the E-Natural in the tenor voice. This eliminates the need for awkward pseudo-cadential motion and allows the passage to stay more comfortably within the mode. Oddly, although this seems like the most obvious solution, one expert I know found it “impossible,” simply because he had learned the piece from Besseler’s edition and, to him, Besseler’s version was the piece. This just goes to show how visceral the reaction to ficta alternatives can be, and reminds us how strange it is not to know for certain what notes were performed at the time a piece was composed.