Plein air painting is a type of performance art, much like music and dance. Performance art is, by its very nature, something that cannot be revised and thus, cannot be "fixed" if something goes wrong. The modern dancer stumbles and falls; the solo pianist loses his place. The dancer doesn't go back and try again, but simply continues. The pianist doesn't flip back a page, but keeps on going.
I've found that it helps in my outdoor painting to take a similar approach. Although, unlike these other script-driven performers, I have an opportunity to exploit my errors and to improvise, if I think of painting as a process instead of as a means to an end, my work inevitably turns out better. And I'm much happier. As I paint, I assign a high value to the act of painting, and not so much to the finished product, which one might consider a souvenir program guide or a memento of the event. Because of this approach, I don't do much "fixing" as I paint. If I do make corrections, they are surreptitious and most likely done later in the studio.
Of course, the patrons and the public assign a high value to this memento and a lesser one -- especially if they aren't witnesses -- to the peformance. And rightly so. I do the same when I've finished painting. The wet panel I hold in my hands is worth so much to me that I carry it in a special box and take great care to not smear the paint. (If not, I'd simply wipe out the painting and use the same panel over and over again!) Once I put down my brush and have a nice memory of my day at Lake Glensevern, I change my perspective, draw back from the act and focus more on the end product. (See accompanying image, "Ice Comes to Lake Glensevern," 8x10, oil. Click on image for a larger view.) The point of considering plein air to be a performance art is this. Like any performer, the more you perform, the better you perform. Granted, you're becoming a better painter under certain circumstances -- within a two-hour time slot because of the changes in sunlight, on a smaller canvas than you might use in the studio, and under the critical gaze of passersby. But you'll paint better under other circumstances, too -- in the studio, at a demonstration, anywhere. You'll just get better, period.