If you are running NetBeans v6.1 or about, you may suddenly get this problem. This is caused by NetBeans inserting two things into your web.xml file. Simply remove/comment them out, and you'll be back in business.
The first one is
<servlet>
<servlet-name>ServletAdaptor</servlet-name>
<servlet-class>com.sun.ws.rest.impl.container.servlet.ServletAdaptor</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
and the other is
<servlet-mapping>
<servlet-name>ServletAdaptor</servlet-name>
<url-pattern>/resources/*</url-pattern>
</servlet-mapping>
IDE - Java - Networking