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 

%%prettify 
{{{
<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

%%prettify 
{{{
<servlet-mapping>
    <servlet-name>ServletAdaptor</servlet-name>
    <url-pattern>/resources/*</url-pattern>
</servlet-mapping>
}}}
/%

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