MSDN reports that 8.4% of all IE9 hangs recently are caused by XMLHttpRequest objects blocking the UI thread because they are using synchronous requests instead of asynchronous. This is a good example of the harm of working against the grain of asynchronous approach in a single-threaded JavaScript environment.
Tim Huegdon uses JavaScript to fix Safari's bug where clicking on a label doesn't set focus to the corresponding field element (a useful accessibility aid). Using YUI and event delegation Tim walks us through the development of this piece of code involving attaching an event listener, getting the target of an event, then finding the field that corresponds to the label, and focusing on it if its a text field, or activating / selecting the form element if its a radio button or checkbox.