No need to set returnValue if preventDefault exists. Thanks kangax for the catch.

src/event.js

@@ -583,9 +583,11 @@ jQuery.Event.prototype = {
 		// if preventDefault exists run it on the original event
 		if ( e.preventDefault ) {
 			e.preventDefault();
-		}
+
 		// otherwise set the returnValue property of the original event to false (IE)
-		e.returnValue = false;
+		} else {
+			e.returnValue = false;
+		}
 	},
 	stopPropagation: function() {
 		this.isPropagationStopped = returnTrue;