[jQuery] AJAX POST causing 500 unimplemented method

[some_random_kid]

I am using an AJAX POST, i.e.:

$.ajax({
     url: serviceUrl,
     async: false,
     data: params,
     type: "POST",
     dataType: "xml",

//more code here...

});

But in the browser, I see that instead of a "POST" I see an "OPTIONS"
as well as a "500 unimplemented method."

Any idea what that "OPTIONS" and "500 unimplemented method" mean?