Let's say, for example, you're writing or improvising variations, and you decide to do something really crazy.
You decide to present the theme in canon with its inversion an octave apart, one line in each hand, adding a 3rd free voice.
This is a tricky business.
There may be moments when these two melodic lines pass very close together, move in similar motion, remain stationary, or sound in octaves.
In cases like this, you may find it necessary to exercise some poetic license and "tweak" the theme with a chromatic inflection or two, or maybe switch the theme to the 3rd middle voice for a few beats, to get things to "fit" [ See blog, Monotony ].
It's perfectly all right to dilute the formal rigor here as long as the result is smooth and beautiful to the ear, which is your guide for everything you do.
It's one more means of "varying" your presentation of the theme, which is the whole purpose of writing or improvising variations.
Let's say, for another example, you're approaching the final chords of a written composition for full organ, you're wanting to produce an even grander effect at the finish than the composer wrote into the music, and the building could stand some acoustical enhancement.
Some organists exercise poetic license in these situations as well and add an extra note, or two, or three, to the harmony with the idea of making a fuller sound at the end that will work most beautifully with an acoustical release [See blog, Acoustical Release].
Taking these types of liberties doesn't really change the harmony of these passages, it merely adds notes that, depending upon your point of view, either boosts the composer's intentions or rewrites it simply to stroke the ego of the performer.
Regardless of how much you choose to employ it, your poetic license has no fee and no expiration date.