Sectoral heterochromia is an eye condition that can make for some interesting and beautiful mutations.
Affecting part of the iris, the affected section usually appears different to the rest of the iris - as seen in the picture on the right. It's generally the result of excess pigment.
It's a fairly uncommon condition that is moreoften seen in dogs and cats, which can also affect humans (albeit not so commonly.) It can be genetically passed down, but it can also occur from disease or injury.