It happened with one cheap Home Depot one I have (that has consistently produced lavender blossoms except once, when it was white tinged with lavender).

I presumed it was just a somatic variation, caused by a mutation in one of the stem cells that produced the branch (and ultimately the blossom). If it hadn't already been October I might have been able to see if it produced more white flowers.

Okay. So I guess it's not entirely unheard of. I'm just glad the flower turned out just as beautiful and the mutation wasn't of the damaging sort.

Reasonably common, actually. I currently have an orange zinnia that has one bright pink blossom on it. The blossom is malformed, however, so the mutation is deleterious--just not nearly enough to kill the plant, the branch, or the flower. I'm idly waiting to see if it produces seeds. It probably will.

Plants have fewer problems with mutations than we do, so it's not at all surprising to find an altered branch, blossom, or petal. In many years of gardening with tons of flowers in 2,000 square feet, I've seen that happen...not often, but perhaps once a year on one plant.

Unusual combinations of genes from tight inbreeding are much more common and occasionally useful; my Salvia splendens is now more the height and color of cardinal flower.

If you like it, you can take a snippet from the branch and try to root it as its own plant.

If you hate it, chop off that particular branch down at the base and it probably won't re-occur. If it does, it probably won't come back next year. If it still does, it's a mutation in the tuber's stem cells and you're stuck with it.