As a customer of mine stands up there SharePoint 2013 farm, they are using least privilege for the service accounts.
During the crawl a large portion of the documents get the following error message in the
Processing this item failed because of an unknown error when trying to parse its contents. ( Error parsing document 'http://sharepoint/Lists/HR Transaction Links/MSS'. Sandbox worker pool is closed.; ; SearchID = 4D01F588-EF0F-4F13-A2AE-49EA0BEXXXXX )
This issue is due to a lack of permissions for the crawl account, the Support team has provided the following steps to resolve the issue
(1) "Start" -> "Administrative Tools" -> "Local Security Policy" (or run: Secpol.msc)
(2) Expand “Local Policies”
(3) Click on “User rights assignment”
(4) Make sure that the search service account has the following privileges
-> "Replace a process level token"
-> "Adjust memory quotas for a process"
(5) Run gpupdate /force
After making that change you can either force a crawl or allow the next scheduled crawl to happen.