In the first image of the cat, which is a work in progress, dark streaks appeared across the left side of the face. Obviously wet paint came into contact with a hand, a sleeve, a rag, something. Instead of ruining the painting these streaks appear to be a part of the painting. They can be viewed as whiskers. A happy accident!
In Vivian case, it was her dog that tried to eat the delicious colors off the paper. A sad accident but not an unhappy ending. Vivian's painting could become a collage!
