When you try to add a managed account to SharePoint 2010, it was throwing a error
“The specified user DOMAIN\SPFASTContentAdmAppPool could not be found. Some or all identity references could not be translated.”
But the user exists in the active directory. When I investigate further I found out that SharePoint does not accept user names which are more than 20 characters in length.
So new rule to the pre-installation documentation. “Manage account ids should be less than 20 characters”.