I was writing a preloader for a project I was working on. The project has a set of slides that it needs to load content for. When each slide starts its preloading, it is supposed to notify up the chain that it is starting, then notify again that it is finishing, before finally resolving the deferred. What I have noticed is that while the first two calls to notify work as expected, subsequent calls do not. Instead, they simply repeat the second call over. This is best demonstrated by example.

Expected output of the following code:

slide_loading
slide_loaded
slide_loading
slide_loaded
slide_loading
slide_loaded

Actual output:

slide_loading
slide_loaded
slide_loaded
slide_loaded
slide_loaded
slide_loaded

Code:

function Slide() {
    this.preload = function() {
        var preload_deferred = new $.Deferred();
        setTimeout(function() {
            preload_deferred.notify('slide_loading');
            preload_deferred.notify('slide_loaded');
            preload_deferred.resolve();
        }, 1000);
        return preload_deferred.promise();
    };
}
var slides = [new Slide(), new Slide(), new Slide()];
var d = [];
for (var i = 0; i < slides.length; i++) {
    d.push(slides[i].preload());
}
var w = $.when.apply(null, d);
w.progress(function(msg) { console.log(msg);});

If you replace the last line with:

w.progress(function() { console.log(arguments);});

You actually get something like the following out:

["slide_loading", undefined, undefined]
["slide_loaded", undefined, undefined]
["slide_loaded", "slide_loading", undefined]
["slide_loaded", "slide_loaded", undefined]
["slide_loaded", "slide_loaded", "slide_loading"]
["slide_loaded", "slide_loaded", "slide_loaded"]