Url for testing:

http://benjaminsterling.com/experiments/bug1.2/animationbug.htm

In the example url, when you do:

$('#moveRightDown').click(function(){
	$('.box').animate({left:400,top:400},100);
	return false;
});

The box will move to left 400 and top 400 when you click the button.

But when you do the following:

$('#moveLeftUp').click(function(){
	$('.box').animate({left:-100,top:-100},100);
	return false;
});

The box will move back 100 left, and 100 top each time you click the button. It looks like the adding the minus sign is causing the issue.