Clausula vera is the most common cadence in two-voiced, sixteenth-century choral writing. The voices either expand to an octave from a sixth or contract to a unison from a third. This type of cadence is heavily prescribed throughout The Study of Counterpoint.
It is done by step, not skip, and as far as I know is not exclusive to raised sevenths in the given mode, though most examples I have seen include altered pitches.
That said, I would like to redact a portion of my original statement as I had a mis-thought. I would like to instead reference and substitute the term musica ficta, which is actually more in-line and appropriate in answer to your original question.
You can read more about Musica Ficta here.
Sometimes when writing in a mode, say, A Aeolian for example, it is necessary to alter pitches in the original mode to either avoid dissonances or to bring about a strong sense of conclusion at the end of a piece or section. Alternatively, a composer might alter specific tones in order to prepare for a modulation, tonicization, or deceptive cadence. Moreover, composers of that time also sometimes considered alteration of some notes as non-functional: only valuable for a shift in musical "color".
Essentially, in this time period, whenever you see an accidental, you can bet musica ficta is happening.
Hope that helps clarify things, and I apologize for the mis-statement in my original answer. I will edit accordingly.