Spawning a zombie pigman close to the portal (or maybe comes from the portal) when we have "mobspawn false" and difficulty 1

It Spawning a zombie pigman when we have mobspawning false and difficulty 1. It have happend to 2 of our portals on our servern since we upgraded it to 1.6.4
And it havent been like that we turned monsters on and then off...
When it happened there was no thunder or any normal pigs around...

Steps to Reproduce:
Don't really know how to reproduce but just turn mobspawning off and have difficulty 1 and be close to a neather portal.