A couple of weeks ago I noticed a weird drop in web usage stats on the site you are browsing now. Kinda weird as the drop was right around Fosdem when usually there is a spike in traffic. 
So before you start.. no I don't preach on practice on my own blog, it's a blog dammit, so I do the occasional upgrades on the actual platform , with backups available, do some sanity tests and move on, yes I break the theme pretty often but ya'll reading this trough RSS anyhow.
My backups showed me that drush had made a copy of the Piwik module somewhere early february, exactly when this drop started showing. I verified the module , I verified my Piwik , - Oh Piwik you say .. yes Piwik, if you want a free alternative to Google Analytics , Piwik rocks .. - I even checked other sites using the same piwik setup and they were all still functional happily humming and being analyzed.... everything fine ... but traffic stayed low ..
This taught me I actually had to upgrade my Piwik too ...
So that brings me to the point I`m actually wanting to make...
as according to @patrickdebois in his chapter on Monitoring "Quis custodiet ipsos custodes?" who's monitoring the monitoring tools, who's monitoring the analytics tools,
So not only should you monitor the availability of yor monitoring tools, you should also monitor if their api hasn't changed in some way or another.
Just like when you are monitoring an web app you shoulnd't just see if you can connect to the appropriate http port, but you should be checking if you get sensible results back from it , no gibberish.
But then again ... there's no revenue in my blog or its statistics :)
