$.ajax modifies the passed in options object. The justification, from the code, is so that it can be checked again later in the test suite.

This seems to be a departure from the normal behavior (and it certainly surprised me that I couldnt pass the same object - with small mods - into multiple ajax calls).

It also seems to be unnecessary, because the extended options object is passed as the this parameter to all the callbacks - so eg beforeSend could be used to examine it.

Here's the code:

ajax: function( s ) {
   // Extend the settings, but re-extend 's' so that it can be
   // checked again later (in the test suite, specifically)
   s = jQuery.extend(true, s, jQuery.extend(true, {}, jQuery.ajaxSettings, s));

I think it the outer extend should be dropped:

   s = jQuery.extend(true, {}, jQuery.ajaxSettings, s);