The error raised for a non-existent command doesn't currently include an error code. For example:

> db.runCommand('foobar')
{
	"ok" : 0,
	"errmsg" : "no such cmd: foobar",
	"bad cmd" : {
		"foobar" : 1
	}
}

Drivers need an error code here to distinguish this case from other failure cases related to the new user manipulation commands (SERVER-6246).

The error code has to remain the same in future releases since driver code will rely on it's value.