Problem is that (according to the man page) smartctl sets bit no 6 if there was an error in the past, so now while everything is alright, the exit code is numeric 64.
https://github.com/munin-monitoring/munin/pull/247
https://github.com/munin-monitoring/munin/blob/devel/plugins/node.d/smart_
最新版は以下の設定で、過去のエラーは無視
[smart_sda] env.ignoreexit 64