Just imagine you're painting a super fine watercolor line adding the last touch of detail to a painting and ... wham-0! A water drop slides down the underside of your brush ferrule and creates this big blurb of color ruining your last bit of detail.
Make sure you dry the ferrule of your brush before creating little detailed areas or just adding a touch of color in a relatively small area. You don't want that hidden water gathering on the underside of the brush ferrule to build up and slide down the bristles to wreak havoc on your painting.
One of the big secrets to successful watercolor painting is water control. So make sure you don't have hidden excess water on the ferrule or even the handle of your brush.