Accredited switching architecture during automatic upgrade
It happened to me because the system used to be an ALIX and is obviously i386/686 only. The configuration was installed on 2.0.2 x64 but the updater config still had the i386 URL. System surprisingly still works except the need to reinstall some packages manually and delete the RRD data.
It would be nice if there was a mechanism to prevent this (in addition to "paying attention to what you are doing")
there are more checks there in 2.1