An artist must be willing to risk everything when creating a work or art. He must be willing to make that additional brush stroke, or draw that new line, or try that other color even though it may possibly ruin the entire work, or at the very least take it in a different direction than what he intended.
No matter how far in advance a work is planned, there are always unknown elements, or unexpected surprises that arise, and which force us make a new decision on a color or element halfway through the painting. It is this very act that is so risky. What if it doesn't work? What if it throws something else off balance? What if it ruins the rest of the painting? But the artist must take the chance. If he doesn't the work will flounder, and he will always wonder, "What if I had done it?" At least for his own peace of mind he must take the chance.
If the new element does ruin the art rather than help, it can sometimes be undone (if you are painting in oils). But many times it cannot be undone (e.g. watercolor). That is just the risk of being an artist, though. No one ever said being an artist was easy.