Ran it to an Error message today flooding a Sitecore 7.1 instance log files with thousands of
Unable to create document builder (). Please check your configuration. We will fallback to the default for now.
Happily this post pointed me in the right direction but my solution was slightly different I needed to add line below to the\App_Config\Include\Sitecore.ContentSearch.Lucene.DefaultIndexConfiguration.config must have been missed out in a previous upgrade.