I noticed that in Firefox 5 and Chrome a callback function to animate() isn't executed if the tab isn't active. But when switched back to the tab the queue of unexecuted callbacks is executed straight away without any delay in between them.

Safari, FF3.6, IE8 and IE9 don't show this bug.

function runMe(){

$('#something').animate({key:val},200, function(){
$('body').append('<p>hello</p>');
});

setTimeout(runMe,1000);
}

runMe();

In the above example it would straight away append 20x "hello"s to the body if switched to another browser-tab for 20s.