I was indexing some external sites with my brand new SharePoint. Fun said I, nay said SharePoint and threw the error "The content processing pipeline failed to process the item." like 3548 times or so.
Add the account running the Search Service to the local admin group. If you are uncertain it's the user that runs the noderunner.exe processes.
I assume this should be done when registering the service accounts and don't consider this as a bug, the search service stores the index on disk when it is being organized and baked before sending it to the database.