wtf.js

$('#calendar .barIcons > .barIcon').each(function (i, icon) {
    var left;
    icon = $(icon);
    
    left = parseInt(icon.position().left, 10);
    alert(left); // increases each time
    return; // Comment this and now the alert is zero each time.
    icon.css({
        "position": "absolute"
    });
});

While this breaks (all lefts are set to zero):

                $('#calendar .barIcons > .barIcon').each(function (i, icon) {
                    $(icon).css({
                        "position": "absolute",
                        "left": $(icon).position().left
                    });
                });

I'm not sure alert(..) is guaranteed to have to run synchronously. I think a browser could be free to defer it to happen at the end of the function. Not positive on that, but it's one way to explain the behavior you're seeing. I've seen this before in Chrome with console.log(..) as well.

(The CSS and structure of the page shouldn't even make a difference - I just can't see how the position().left could POSSIBLY be affected by the position: absolute; it's value must have been resolved BEFORE the position absolute is applied.)

@getify yeah I know console.log can be async - didn't think alert could. But even then, it is actually setting the values to zero too, even without the alert.