The "Prevent memory leaks in IE" code block attaches a function to clean up events on window unload. Since browsers don't guarantee the order of event handler execution, this code can sometimes run before other window unload handlers and remove them before they have a chance to run.

There is a comment that states "// Window isn't included so as not to unbind existing unload events". This is true for jQ 1.3.2, but is missing for 1.4.2.

The 1.3.2 code to do this excluded the first item in jQuery.event, the handle of which was always the window object. The handle is not the window object in 1.4.2.

So it makes sense that this disappeared, however, the comment stayed and the code doesn't currently reflect that.

A proposed fix starts at line 2608 of jQuery 1.4.2.

Change this:

// Prevent memory leaks in IE
// Window isn't included so as not to unbind existing unload events
// More info:
//  - http://isaacschlueter.com/2006/10/msie-memory-leaks/
if ( window.attachEvent && !window.addEventListener ) {
	window.attachEvent("onunload", function() {
		for ( var id in jQuery.cache ) {
			if ( jQuery.cache[ id ].handle ) {
				// Try/Catch is to handle iframes being unloaded, see #4280
				try {
					jQuery.event.remove( jQuery.cache[ id ].handle.elem );
				} catch(e) {}
			}
		}
	});
}

To this:

// Prevent memory leaks in IE
// Window isn't included so as not to unbind existing unload events
// More info:
//	- http://isaacschlueter.com/2006/10/msie-memory-leaks/
if ( window.attachEvent && !window.addEventListener ) {
	window.attachEvent("onunload", function() {
		for ( var id in jQuery.cache ) {
			var item = jQuery.cache[ id ];
			if ( item.handle ) {
				if ( item.handle.elem === window ) {
					for ( var type in item.events ) {
						if ( type !== "unload" ) {
							// Try/Catch is to handle iframes being unloaded, see #4280
							try {
								jQuery.event.remove( item.handle.elem, type );
							} catch(e) {}
						}
					}
				} else {
					// Try/Catch is to handle iframes being unloaded, see #4280
					try {
						jQuery.event.remove( item.handle.elem );
					} catch(e) {}
				}
			}
		}
	});
}

Demo:

http://jsbin.com/uquwu3