Return control of $ and jQuery only if it is really necessary. This make

jQuery.noConflict() callable anytime.

src/core.js

@@ -373,9 +373,9 @@ jQuery.extend = jQuery.fn.extend = function() {
 
 jQuery.extend({
 	noConflict: function( deep ) {
-		window.$ = _$;
+		if ( window.$ === jQuery ) window.$ = _$;
 
-		if ( deep ) {
+		if ( deep && window.jQuery === jQuery ) {
 			window.jQuery = _jQuery;
 		}