If you run an application such as HelloTooltip on the Mac and close the window with the red window-close button while the tooltip is still visible, you will get the following exception. I cannot reproduce this on Windows.

Glass detected outstanding Java exception at Java_com_sun_glass_ui_mac_MacWindow__1close:/Users/kcr/javafx/2.1-kcr-gfx/jfx/glass/glass-mat-lib-macosx/src/com/sun/mat/ui/GlassWindow.m:1267
Exception in thread "JavaFX Application Thread" java.lang.IllegalStateException: The window has already been closed
at com.sun.glass.ui.Window.checkNotClosed(Window.java:200)
at com.sun.glass.ui.Window.ungrabFocus(Window.java:542)
at com.sun.javafx.tk.quantum.WindowStage.ungrabFocus(WindowStage.java:597)
at javafx.stage.Window.impl_decreaseFocusGrabCounter(Window.java:995)
at javafx.stage.PopupWindow.impl_visibleChanged(PopupWindow.java:347)
at javafx.stage.Window$13.invalidated(Window.java:749)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:108)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:142)
at javafx.stage.Window.setShowing(Window.java:764)
at javafx.stage.Window.hide(Window.java:789)
at javafx.stage.PopupWindow.hide(PopupWindow.java:297)
at com.sun.javafx.stage.WindowPeerListener.closed(WindowPeerListener.java:111)
at com.sun.javafx.tk.quantum.GlassWindowEventHandler.handleWindowEvent(GlassWindowEventHandler.java:68)
at com.sun.glass.ui.Window.handleWindowEvent(Window.java:934)
at com.sun.glass.ui.Window.notifyDestroy(Window.java:849)
at com.sun.glass.ui.mac.MacWindow._close(Native Method)
at com.sun.glass.ui.Window.close(Window.java:213)
at com.sun.javafx.tk.quantum.WindowStage.close(WindowStage.java:575)
at javafx.stage.Window$13.invalidated(Window.java:743)
at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:108)
at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:142)
at javafx.stage.Window.setShowing(Window.java:764)
at javafx.stage.Window.hide(Window.java:789)
at com.sun.javafx.stage.WindowCloseRequestHandler.dispatchBubblingEvent(WindowCloseRequestHandler.java:45)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:38)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:37)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:53)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:33)
at javafx.event.Event.fireEvent(Event.java:171)
at com.sun.javafx.stage.WindowPeerListener.closing(WindowPeerListener.java:104)
at com.sun.javafx.tk.quantum.GlassWindowEventHandler.handleWindowEvent(GlassWindowEventHandler.java:71)
at com.sun.glass.ui.Window.handleWindowEvent(Window.java:934)
at com.sun.glass.ui.Window.notifyClose(Window.java:840)