Recently, I faced one strange issue on my SharePoint farm. My documents were not crawling and they were showing the below error.

Error

The Content Processing pipeline failed to process the item. (Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index; ; searchID=XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX)

So after some analysis, I learned that this line comes up when some required managed properties in the search schema are set to ‘Allow Multiple Values’.

To find which managed properties are causing the problem, just log in to your application server where the search is configured and open the recent ULS log and do a keyword search for “Content processing”. You can find the log line like below.

NodeRunnerContent1-1af8a817-055 (0x142C) 0x2934 Search Content Processing awyjr Monitorable “PropertyMapperProducer[[Microsoft.CrawlerFlow-4c7cf676-1194-43d3-b176-a4b39c6fedec]]: Failed to map values to field Keywords. Metadata: OutputAsList True, Type String, RemoveDuplicates True” dd750643-71e0-4d79-962c-6e47c472d02d

Finally, in my case, “Keywords” managed properties was causing the problem, so I went to my search administration page and search schema and searched for “keywords” managed properties

Then, I removed Multi from the managed properties and ran the full crawl. Then, the above-mentioned error disappeared and all documents crawled successfully.
These troubleshooting steps cleared all my “Content Processing pipeline failed” errors and I don’t get those errors again.