I shall begin this article by boldly, far-fetchedly assuming that you know that the word stain means. In case you're confused, here's a direct quote from the Wikipedia page for "stain":
A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials.
Was that what first popped into your head when I assumed you know what a stain is? No? Did you, like me, automatically think a stain is something negative - a white shirt ruined by tomato sauce - a splash of a wrong color in canvas - coffee rings on a mahogany table?
The word discoloration does by no means include a moral stand; it does not state if its existence is wrong or right. It only means there's a difference between the area where the chemical interaction takes place and the area where it does not. Meaning, you can interpret this as all art where you paint or draw something is, in its essence, the practice of discoloration - staining some parts if not all of the canvas or paper with the materials of your choosing that induce the chemical reaction.
And even if the staining act was accidental, it does not automatically mean the result is an undesired one. Some accidents are happy right from the get-go. Some accidents seem mistakes in the beginning, yet yield unexpected and wonderful consequences. And some accidents, even if they aren't beneficial or beautiful from the start or end up making something better, can be seen as art or positive because they allow you to see the world differently, or force you to deal with failure, or push you to think outside the box.
Whether a stain is a good or a bad thing depends solely on the way you react to it.
Have you found or caused a stain recently? Did something come out of it? In your everyday life, are you constantly careful not to stain things, or do you operate naturally and perhaps clumsily and check the damages later? Do you let stains rest, or do you try to eliminate them the moment you see they exist?