"Fatal error: Call to undefined method stdClass::add_query_var() in .../wp-includes/taxonomy.php on line 356"
An Admin of an "under construction" web site is using one of my plugins, as well as a number of others, and started getting this error message right after she upgraded versions of WordPress, jonradio Multiple Themes and NextGen Gallery (NGG). The error remains when she deactivates all other plugins, but disappears if either NGG or my plugin is deactivated.
You'll find more details in this thread:
I've run out of ideas. And could really use some advice.