How do I troubleshoot 'Theme key "" not found'? How do I troubleshoot the error Theme key "" not found?

I found dozens of search results regarding specific keys, but nothing useful for debugging an empty key ("").

Here is the full entry from syslog:

Sep 12 09:45:47 core drupal: http://www.example.com|1347457547|theme|192.168.1.10|http://www.example.com/|http://www.example.com/admin/config/development/logging|1||Theme key "" not found.


What exactly triggers this error and how should I go about debugging it?
This warning message comes from theme() function in includes/theme.inc. For temporary debug you could just add ddebug_backtrace() there (if you're using Devel module) to see where the call comes from (and see what are the params). If not using Devel, just check output of debug_backrace(). And then it all depends what you are going to find out...