--- old/modules/graphics/src/main/java/javafx/stage/Stage.java	2015-05-15 12:02:19.753114100 -0700
+++ new/modules/graphics/src/main/java/javafx/stage/Stage.java	2015-05-15 12:02:18.923113000 -0700
@@ -456,6 +456,10 @@
             throw new IllegalStateException("Stage already visible");
         }
 
+        if (!Toolkit.getToolkit().canStartNestedEventLoop()) {
+            throw new IllegalStateException("showAndWait is only allowed only while handling system events");
+        }
+
         // TODO: file a new bug; the following assertion can fail if this
         // method is called from an event handler that is listening to a
         // WindowEvent.WINDOW_HIDING event.