In NC, a lot of republicans didn't reregister as democrats because of a large number of important republican primary races. Plus they had to do so by April 11th. Nearly a month ago. Obama ended up winning by 14%, but had a greater than 20% lead in the polls shortly before primary day.
Considering that she had been way ahead in the polls in Indiana, it would seem that OC backfired and caused some Dems who were going to vote for Hillary to switch and vote for Obama. Seems like the same may have happened in NC, since Obama won by a larger margin than was expected.