This is the first of a series of blog posts that deals with Plone migrations.
During the migration of my company site zopyx.com from Plone 3 to Plone 4 I came across some issues with the migration tool where the migration tool tried to do something (no idea what) with non-existing content-objects (obviously stale catalog entries)....so portal_migration never terminated with success.
So I tried to be smart and cleared the portal_catalog (Advanced tab of the portal_catalog -> Clear catalog) first. After that I started the migration again and it finished successfully. After that I went to portal_catalog again and used Clear and Rebuild to re-create the portal_catalog from scratch. All content showed up as expected. Not sure if this is a general good advice but perhaps it might help in some situations with migration issues.