1) somatic mutations: http://www.nature.com/scitable/topicpage/somatic-mosaicism-and-chromosomal-disorders-867
Mutations happen all through the life of an organism. If that mutation happens in a germ cell (sperm or egg) the mutation is passed down to every cell in the next generation. However, if the mutation occurs in a somatic cell (non-germ cell), then when this cell divides (during growth) all the resulting cells carry the mutation.
In chimera (similar, but slightly different) you can have a person with one blue eye and one brown eye.