I previously blogged a solution for the Unknown Flavour error that involved modifying the blosxom.cgi script itself. I recently found my modification caused requests for the RSS 0.91 feed and atom feeds to return the default flavour instead of the desired feed. I suspect any plugin using a flavour that inserted itself into the Blosxom template hash table without corresponding flavour file(s) probably didn't work correctly after my modification.
While investigating a fix for that bug I noticed that a better solution would be to override the Blosxom template subroutine. So I created the defaultflavour plugin. This plugin fixes the bug by first applying any registered flavour templates if the requested template files are not found. If that registered flavour templates don't exist the plugin finally applies the default flavour template files.
Download version 0.1 of the defaultflavour plugin here.
No configuration of the plugin is required. Just copy it into the plugin directory.