There is no evidence that anyone ever hatched the wrong region-specific pokemon from an egg.
But i personally have plenty of evidence that plenty of people here in Munich are hatching Mr. Mime from eggs. On average, all the higher level players got one or two of them from eggs. I have seen some people having other region-specific ones, one got it via GPS-hacking and got banned later, all the other ones actually traveled into another region and got it that way.
So all the evidence points in one direction, and only rumours that somehow persist in spite of being so unreasonable claim the opposite.
EDIT: Oh and make sure to keep a few of your region-specific pokemon, if the trade function ever comes, these will be really useful and worth an awful lot if you travel to a different region.