While Swing has never claimed to support painting from mutliple threads, at the same time, there appear to be a handful of apps relying on this behavior.
Here are problematic traces we've seen:

    Exception in thread "Thread-3" java.lang.NullPointerException
    at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:276)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1055)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5042)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4860)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4872)
    at com.sun.pdasync.SyncUI.SyncTabs.updateProgress(SyncTabs.java:142)
    at com.sun.pdasync.SyncUI.PDASyncDlgUI.updateProgress(PDASyncDlgUI.java:256)
    at com.sun.pdasync.SyncUI.PDASyncDlgUI.updateViewer(PDASyncDlgUI.java:348)
    at com.sun.pdasync.Logger.Logger.fireEvent(Logger.java:123)
    at com.sun.pdasync.Logger.Logger.notifyListeners(Logger.java:60)
    at com.sun.pdasync.Logger.Logger.doProgressUpdate(Logger.java:182)
    at com.sun.pdasync.HotSync.HotSync.CreateTaskMaster(HotSync.java:894)
    at com.sun.pdasync.HotSync.HotSync.PreLocalSync(HotSync.java:2331)
    at com.sun.pdasync.HotSync.HotSync.LocalSync(HotSync.java:3007)
    at com.sun.pdasync.HotSync.HotSync.PerformHotSync(HotSync.java:1198)
    at com.sun.pdasync.HotSync.HotSync.connectionSniffer(HotSync.java:1695)
    at com.sun.pdasync.HotSync.HotSync.pollConnection(HotSync.java:1728)
    at com.sun.pdasync.CommPort.SerialPortCommunicator.fireEvent(SerialPortCommunicator.java:130)
    at com.sun.pdasync.CommPort.SerialPortCommunicator.serialEvent(SerialPortCommunicator.java:422)
    at com.sun.comm.SolarisSerial$SSCheckStatusThread.RaiseEvent(SolarisSerial.java:585)
    at com.sun.comm.SolarisSerial$SSCheckStatusThread.run(SolarisSerial.java:432)

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    Exception in thread "Thread-3" java.lang.NullPointerException
    at javax.swing.JComponent.getComponentGraphics(JComponent.java:708)
    at javax.swing.JComponent.paint(JComponent.java:959)
    at javax.swing.JComponent.paintToOffscreen(JComponent.java:5094)
    at javax.swing.BufferStrategyPaintManager.paint(BufferStrategyPaintManager.java:279)
    at javax.swing.RepaintManager.paint(RepaintManager.java:1055)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5042)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4860)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4872)
    at com.sun.pdasync.SyncUI.SyncTabs.updateProgress(SyncTabs.java:142)
    at com.sun.pdasync.SyncUI.PDASyncDlgUI.updateProgress(PDASyncDlgUI.java:256)
    at com.sun.pdasync.SyncUI.PDASyncDlgUI.updateViewer(PDASyncDlgUI.java:348)
    at com.sun.pdasync.Logger.Logger.fireEvent(Logger.java:123)
    at com.sun.pdasync.Logger.Logger.notifyListeners(Logger.java:60)
    at com.sun.pdasync.Logger.Logger.doProgressUpdate(Logger.java:182)
    at com.sun.pdasync.HotSync.HotSync.DoSync(HotSync.java:2943)
    at com.sun.pdasync.HotSync.HotSync.DoConduitSync(HotSync.java:2977)
    at com.sun.pdasync.HotSync.HotSync.LocalSync(HotSync.java:3012)
    at com.sun.pdasync.HotSync.HotSync.PerformHotSync(HotSync.java:1198)
    at com.sun.pdasync.HotSync.HotSync.connectionSniffer(HotSync.java:1695)
    at com.sun.pdasync.HotSync.HotSync.pollConnection(HotSync.java:1728)
    at com.sun.pdasync.CommPort.SerialPortCommunicator.fireEvent(SerialPortCommunicator.java:130)
    at com.sun.pdasync.CommPort.SerialPortCommunicator.serialEvent(SerialPortCommunicator.java:422)
    at com.sun.comm.SolarisSerial$SSCheckStatusThread.RaiseEvent(SolarisSerial.java:585)
    at com.sun.comm.SolarisSerial$SSCheckStatusThread.run(SolarisSerial.java:432)