[Someone]

"Chimerism is a possible explanation but vanishingly unlikely. Two identified cases, ever (i.e. none before this)."

http://en.wikipedia.org/wiki/Chimera_(genetics)#Humans gives an earlier case and references http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.149..., which states blood type chimerism occurs in 8% of twins, 21% of triplets.

So, chimerism in the broad sense isn't that rare. I have no idea whether that is different from what is described here, but I would think it isn't. Those blood cells have to come from somewhere.

Chimerism also occurs as a result of organ transplants.