I have upgraded both WordPress and feed-json plugins.
Each time I use feed (but not any other) URL, I see in apache error logs file:
PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, first array member is not a valid class name or object in /var/www/website/wp-includes/plugin.php on line 470
I checked already, it's because of do_feed_json(). Can you fix this for later version?
Please, let me know if you need more details.
* WordPress: 3.9.1
* Feed Json: 1.0.9