I've just been debugging an issue in my software where adding and removing classes with addClass and removeClass were doing something slightly odd. I realised that while addClass will add multiple instances of a class to an element (fair enough, I've asked it to do that), removeClass will only remove the first instance of a class. I would have expected it to remove all instances of the given class (although this could be an incorrect assumption on my part). For example

$("#test1").addClass("disabled disabled");

alert("test1 class after addClass ='"+$("#test1").attr("class")+"'");

$("#test1").removeClass("disabled");

alert("test1 class after removeClass ='"+$("#test1").attr("class")+"'");

The first alert will give "disabled disabled" while the second alert gives "disabled". I had anticipated it just being an empty string.