When your fresh brush stroke hits a still damp wash it will force the original wash out in a irregular, often fractal manner. This can totally screw up what you are intending to do, unless you do it intentionally. Practice playing with paint and coping with "happy accidents."  In watercolor this term applies to the movement of pigment through a water wash to create a "wet in wet" effect. It also applies to the migration of paint through any painting surface.