Deleting jsonp and jsonpCallback from the repeated request fixes it:

http://jsfiddle.net/fMt3t/10/

Likewise, setting jsonp and jsonCallback to null on the original request fixes it:

http://jsfiddle.net/fMt3t/11/

Lastly, using jQuery.ajaxSetup() to remove jsonp and jsonpCallback

http://jsfiddle.net/fMt3t/12/

Your json request gets promoted to a jsonp request the second time around because you now have the jsonp and jsonpCallback options from ajaxSettings in there.

http://jsfiddle.net/r4juA/1/

But why are those options being added? And regardless of them being there, it shouldn't be using them since the dataType is json, not jsonp, right?

These options are added because the options map you're getting within the callbacks is the fully constructed, internal, one (including the options provided to ajax AND the options in ajaxSettings).

And the json request gets promoted as a jsonp request because ajax allows stuff like:

$.ajax({
    url: "...",
    dataType: "json",
    jsonp: "callbackParameter"
});

// OR

$.ajax({
    url: "...",
    dataType: "json",
    jsonpCallback: "myCallbackName"
});

I could live without it, to be honest, but it made the transition from 1.4.x to 1.5.x for compatibility reasons.

This appears to have been fixed as of 1.6.4: http://jsfiddle.net/fMt3t/80/