It is possible, although uncommon, for two blue-eyed parents to have a brown-eyed child. This is due to the mutation of the HERC2 gene, which is responsible for blue eyes, and its relationship to the melanin-producing gene OCA2. Both must be functioning for the production of pigment.
A functioning HERC2 gene means that the OCA2 gene produces pigment if it is not already disabled through inheritance, thus ensuring that the child has brown eyes. Meanwhile, if the HERC2 is disabled but the OCA2 is not, the result is blue-eyed offspring. The opposite circumstances produce the same result. The relationship between these genes allows someone to be a carrier of the brown-eyes trait without having brown eyes.