You don't see fasciation often on deciduous plants but it happens. It happens quite a bit on conifers and on cactus and other succulents, ferns too. Happens a lot with flowers. Most of the time they will not form roots when rooting is attempted so they are normally grafted. The Japanese fantail willow is an example of one that works and is a successful plant, measuring success as desirability. As are celosia that are planted as annuals. While it can be a random mutation, it can also be caused by bacterial or viral infections, thus it is not encouraged except where a beneficial or desired plant is produced.
People with 'webbing' between their fingers and toes sometimes lack the same or a similar abscisic acid enzyme. In humans this is sometimes caused by a genetic defect, rarely by environmental factors (although; epigenetics, this is a debatable thing).
All multicellular organisms grow taller and larger from a single clump of cells (embryo or later in the case of plants one or more meristems). These cells need to be cleaved enzymatically to grow into organs and limbs. ABA is one of those enzymes, but there are a couple hundred more at least. If said organism lacks a certain cleaving enzyme, or the organism doesn't produce it for whatever reason, the organs develop as a whole, uncleaved. In most plants this is often temporary and usually localized (functioning is not optimal and plants tend to drop less functional parts off to benefit high functioning parts, aside from stopping infection through necrosis - ironically by cleaving the cells enzymatically off and shutting their sap flow down).
In humans, this mutation/defect can be pretty lethal because the body isn't able to remove malignant growths, so even though these people look very normal in most senses, they require regular checkups to make sure there isn't something growing that the body should've gotten rid of.
Yeasts, bacteria and more primal singular cells need these enzymes to divide themselves after budding, sporulating or copying themselves. There are some theories that the basis of multicellular life was laid out by a mutation that stopped these single-celled bugs from dividing into two, and growing into a larger multicellular organism.