The validateNS() routine in src/mongo/db/dbcommands_admin.cpp that does the work of the 'validate' command has an array called 'errors' that it will return once the command has finished. This array is supposed to contain a list of the validations that failed when the command returns 'valid' : false.

If the first extent in the collection fails the Extent::validates() test or if the first extent has a non-null 'xprev' pointer, 'valid' will be set to false but no explanation will be added to the 'errors' array.