Munin
From Briki
Error for checks involving the word 'root'
You may see errors in /var/log/munin/munin-graph.log of the form:
... : opening '/var/lib/munin/--g.rrd': No such file or directory
This sometimes occurs because the particular monitor returns a category of "root", which is a reserved word in munin. To fix this, you need to patch a few munin files, as described here: http://munin-monitoring.org/ticket/794.
I noticed this issue for cpubyuser, and lvm_myhost (since I had an lvm volume called "root").