Multi page schemas
In Plone 3.0, all fields from all schematas will be loaded at once. If you depend on your schematas (fieldsets) to be processed one page after the other, you'll need to mark your Archetypes content type that uses it (not the schema itself) with the IMultiPageSchema interface.
The interface lives in Products.Archetypes.interfaces.IMultiPageSchema. The code to mark your content type would look like this:
from zope import interface from Products.Archetypes.interfaces import IMultiPageSchema # ... interface.classImplements(MyContentType, IMultiPageSchema)