Denny,
I found something and I validated on my local instance(AEM 6.2). Basically, when we enable a renderer for GET servlet, we see this behavior. Surprisingly, I see a whole lot of live sites having this issue and worst thing is these pages are getting indexed by google and showing up in search results.
Anyway, there is a OSGI configuration in felix console which needs to be tweaked to fix this issue. Please follow these steps
1) Please go to felix console's configuration manager http://<hostname:<port>/system/console/configMgr
2) Go to Apache Sling GET Servlet and edit the configuration values
3) You will see a "Enable HTML" option , if the check box is checked/selected, it means that you are enabling the HTML renderer for the default GET servlet and that produces the result you are currently witnessing.
4) Disable(unselect the check-box) this option and try it again, you should see an expected 404 page instead of the "Resource dumped by HtmlRendererServlet" message.
By default, I was not getting this error/message on my local instance, but I reproduced it by enabling the "Enable HTML" option
Hope this helps!!