Mistakes. We hate that word, don't we? When we goof up, our first impulse is to berate ourselves and get angry.
But what is a mistake? It's merely a stutter in the learning process; a glitch that can either make you bitter and give up, or, cause you to calm down, reassess, and proceed in a straighter course.
Sometimes, mistakes are "happy accidents," a phenomenon which often happens in the act of painting. You can be going along, really lost in your creative space, and suddenly, your brush seems to take on a mind of its own. At first, you think: What? But then, you step back and see the perfect, unplanned stroke that is just what that few square inches of canvas required to be finished.
Trust your brush; at least till you've given yourself time to look at the work with a fresh eye.
Of course, even bad mistakes are teachers. Just yesterday, I was touching up an as yet un-exhibited painting when I saw these small, regular circles all over the sky! At first I couldn't imagine how they got there. It was only after I'd repainted the clouds that I figured it out: I had wrapped the painting in bubble wrap before it was completely dry, and pressure against the wrap caused it to make impressions on the slow drying white paint. Now that was a very important lesson. Imagine several paintings newly hung at a show with little circles all over them? Aaaarrgghhhh!