contentType header attribute occasionally not present in ajax web method

Question

Hi All,

I am having a simple web service method which returns JSON serialized response. The attribute to web method is

[WebMethod, ScriptMethod(UseHttpGet = true, ResponseFormat = ResponseFormat.Json)]

As security of the return data is not a problem, I am Get enabling the HTTP response.

On the client (browser) end, my code is as below

    //Add webservice and generate javascript proxy classes.
    ScriptManager manager = ScriptManager.GetCurrent(Page);
    ServiceReference reference = new ServiceReference("/Services/xyz.asmx");
    if(manager == null)
    {
        manager = new ScriptManager();
    }
    manager.Services.Add(reference);

Everything works smoothly but occasionally on the web server's event viewer, I see the following error

"Request format is unrecognised for URL unexpectedly ending in /SomeWebServiceWebMethod"

After fiddling through the request headers using Fiddler, I figured out that if I make direct request to the webservice link, contentType header attribute is missing and that is what throws the error.

There is a good post by scott guthrie explaining this.

I am not able to replicate the issue and not sure what removes the contentType attribute from the request header occasionally.

Your help will be highly appreciated.