I had to do something a few days to ago to correct some errors in the
history (I had an invalid email address configured).
The result of this is many of the commits towards the top got
"rewritten". This means nothing major unless you were trying to
update a checkout, and if that happens, you may just want to
work from a clean new checkout.
This also impacted about 5 or 6 pull requests, as what github makes
available unfortunately has a bit more metadata or something than just
a basic patch, so it made some things hard
to apply. I've asked those folks if they can help me out by trying to
reapply their changes -- which is unfortunate.
Moral of the story is I git does not validate that email addresses are
RFC compliant and it is very annoying if you have to use
git-filter-branch 
Sorry for any confusion this caused.
--Michael