Upon visiting my Piwik homepage today I got a page that said "an error occurred" followed by "Piwik couldn't write to some directories (running as user 'www-data')." and a list of commands to try.

I have monitoring in place that checks whether HTTP services respond with 200 OK, aka a success status code. Obviously Piwik is not OK in this scenario, and should return 500 Internal Server Error instead.