For those of you who paint with watercolours, the disappointment of smudging wet paint or dropping a spot of paint in an area that was meant to be either unpainted or left plain will be well understood.
I remember dropping a blob of black paint on a proposed plain area of a painting of two Black Cockatoos. The drop was about 10mm diameter. I was a little taken aback by it to say the least. In that case I was able to add leaves to the composition and it went unnoticed to other people: but not for me. I was disappointed because the composition was adversely affected. For awhile I experimented with plain paper overlays but that interfered with the development of the painting.
To prevent a recurrence of that event, I now lay a sheet or part of a sheet of tracing paper over the picture. This has delivered a few benefits: