I came across the following error in the crawl logs of a SharePoint 2013 instance.
“The content processing pipeline failed to process the item. ( Object reference not set to an instance of an object. )”
In my case this turned out to be because I had deleted the managed metadata service that had been originally created for this farm. This was causing the existing search service to attempt to reference terms in the old managed metadata service and failing.
Recreating the search service solved this issue for me.
