//Fix of adding action at the same priority to prevents corruption of the $wp_filter array while it's looping.
add_action('init','WPECfix', 8000000);
function WPECfix(){
        if ( $priority = has_action('pre_get_posts', 'wpsc_generate_product_query') )
                add_action( 'pre_get_posts', '__return_null', $priority );
}