This is truly a fantastic new plugin. As it peeks very deep into the WP processes it can be expected to have some incompatibilities with other plugins. I've found two further plugins which will fail if Query Monitor is enabled.
First is "FirePHP / Firebug PHP Integration". It results in a fatal error:
[22-Nov-2013 15:54:46 UTC] PHP Fatal error: Call to a member function get() on a non-object in /usr/www/xxxxx/wp-includes/cache.php on line 113
Cache plugin W3TC is enabled but should not cache anything as I'm logged in as a administrator.
Second is plugin "Infinite Scroll". Calling an URL in the browser is no problem. But it loads the follow page using a XMLHttpRequest. This request fails with a
Status-Code 500 Internal Server Error. I've checked the processing of the XMLHttpRequest on server side and found that the problem must occur in function wp_footer().
Infinite Scroll executes some code in the footer hook to transfer configuration values to the JS. I suspect that this may be the reason for the failure.
Request-URL: http://myserver.com/page/3/ Request-Methode: GET Status-Code: HTTP/1.1 500 Internal Server Error X-Requested-With: XMLHttpRequest Referer: http://myserver.com/page/2/