Those sorts of quirky things do occasionally happen with iris blooms. It's usually associated with weird climatic conditions, but anything that interferes with bud formation can cause it. It probably won't happen again to that plant, though it could.

Laurie

I have one deep purple bearded iris with about 6 inch stems which blooms "quartered" (four of everything) and even one bloom with five standards and falls. It is the same one every year. Can't be because of bad weather - we had a mild winter with deep snow cover this year.