We're of two minds about the whole thingChimeras are the most extreme form of Siamese twins; two twins who have fused into a single person. How this happens is still a slight mystery; either they are two separate embryos that fuse as Siamese twins do into a single embryo, or the slightly more disturbing idea that one twin consumes the other, gaining those characteristics. (Keep in mind this would be happening within the first few weeks of pregnancy, so don't worry about it too much.)
Like "Siamese Twins" have the term "Conjoined Twins", Chimeras are also known as "Disappeared Twins". We're rooting for Chimera, though.
It is thought that many Chimeras don't even know what they are and live their whole lives not knowing. The situation usually comes to light in conjuction with a medical procedure, say an organ transplant or an invasive operation like a gall bladder removal. Then the usual tests are taken but some really whack-ass results emerge: multiple blood types, structures of male and female organs inside the body (hermaphrodism), and organs that are incompatible with the children of the chimera, where normally they should be. One of the more interesting obvious signs are eyes of different color.
In November of 2003, a 52-year-old woman in Boston was found to be incompatible with two of her three children for an organ transplant, meaning she had lived an entire half-century with no idea of her condition. Thankfully, this is one freak act of nature that allows the individual to lead a "normal life" throughout.