the development between ability, action, and accident toward a delighted outcome
There isn't a definition that I found for the words painters use to describe the happening of a happy accident. I'll describe why I came up with my definition.
First, ability. There has to be some kind of ability or aptitude that the person is putting forth, some starting point for the process.
Second, action. It's in the doing that there is opportunity for discovery.
Third, accident. In the process there is some expectation from the deliberate ability and action. In the accident there is a misstep but isn't defined as a mistake because of the fortunate or lucky outcome.
In my research I think 'happy accident' is similar to the original meaning of serendipity, the 1557 definition, not today's definition.
Serendipity: 'were always making discoveries by accidents and sagacity, of things they were not in quest of'
Today's definition doesn't really take into account the sagacity of serendipity. Today serendipity is just a chance discovery with a chance event without any deliberate effort. Despite most artists possibly not being able to define it they definitely know it when they see and feel it. Though I can't remember a specific painting I know I have had it happen.