In the Plant Swap with my sister last year, one of my favorite acquisitions was caryopteris. (At the time it was known as the "mystery purple shrub" but it was later properly identified.) I planted it in the front right border and it grew throughout the season. However, it never turned out the beautiful purplish-blue blooms that I expected, and I was puzzled. But I still had faith.
This year, the plant grew taller and was yellow once again. I began to grow skeptical, particularly when I noticed waves of a suspiciously similar looking plant in the roadside wilds on my morning run.
The impossibility of this actually being caryopteris was confirmed when I discussed the mystery with both of my sisters recently. One had researched the plant and found it is a shrub and therefore you can't divide from cuttings or portions of the plant.
So here stands the mystery plant, which I now consider a weedy intruder in an unfortunate (but unintentional) case of "the old bait and switch" -- and it won't remain here much longer!