Adding a new web service to an application today so that I can delay loading some web user controls and ran into this 500 Error when calling the web service:

Request format is unrecognized for URL unexpectedly ending in /GetHtml

Found this KB article with the fix:

Add the following to web.config since GET and POST are disabled by default in ASP.NET 2.0 and greater:

<configuration>

    <system.web>

    <webServices>

        <protocols>

            <add name= "HttpGet&quot">

            <add name= "HttpPost">

         </protocols>

     </webServices>

   </system.web>
</configuration>