This isn’t to diss Bob Ross, because he’s a delight. But like everything in art, there is nuance and alternative meaning. When Bob talked about happy accidents, he was teaching his viewers not to break the flow of their work with thoughts about how they messed up. It’s a way of reframing the unforeseen.
Mistakes will happen. But whether your work is meticulously planned or completely spontaneous, it can be helpful to keep rhythm with them. It’s another Zen or Taoist concept applied to creation: it doesn’t matter if errors crop up, because they become part of the humanity of your work. It’s only more real for small flaws.
And sometimes they can take us in different directions we hadn’t thought of, or would never find in a perfect thing.