If copydb is used to copy from one database to another on the same machine and access control is enabled, it will fail every time with the error:
db.getSiblingDB("admin").runCommand( { copydb: 1, fromdb: "mySourceDB", todb: "myTargetDB" } ) { "ok" : 0, "errmsg" : "index query on ns mySourceDB.system.namespaces failed: not authorized for query on mySourceDB.system.namespaces" }