There's also a variation on this, called a "mosaic", in which one of the cells in the developing embryo underwent a mutation (such as a copy error, etc.), resulting in a person who has some tissues/organs (i.e., those which developed from the original mutated cell) having a genetic variation compared to the other parts of the same body.
This can lead to odd medical disorders, whereby some tissues in the body are susceptible to a particular problem while others aren't. It can also make DNA screening for genetic illnesses difficult because the cells (or blood) sampled for genetic screening may be the "healthy" ones.
There is speculation that some cases of Alzheimer's or autism may be a result of this.
There's also a variation on this, called a "mosaic".
This I have seen in someone who was a Mosaic Downs baby.