FULL PRODUCT VERSION :
java version "1.6.0-rc"

A DESCRIPTION OF THE PROBLEM :
There are classes in Swing whose method's declaration includes the synchronization keyword. Given that Swing is single threaded these methods should have the synchronization keyword removed.

The following classes are potential candidates:
javax/swing/AbstractAction.java
javax/swing/ActionPropertyChangeListener.java
javax/swing/BoxLayout.java
javax/swing/BufferStrategyPaintManager.java
javax/swing/DebugGraphicsObserver.java
javax/swing/GroupLayout.java
javax/swing/ImageIcon.java
javax/swing/JApplet.java
javax/swing/JCheckBoxMenuItem.java
javax/swing/JComponent.java
javax/swing/JEditorPane.java
javax/swing/JInternalFrame.java
javax/swing/JLayeredPane.java
javax/swing/JTabbedPane.java
javax/swing/JTable.java
javax/swing/JTextField.java
javax/swing/KeyStroke.java
javax/swing/LayoutStyle.java
javax/swing/MenuSelectionManager.java
javax/swing/MultiUIDefaults.java
javax/swing/PopupFactory.java
javax/swing/ProgressMonitorInputStream.java
javax/swing/RepaintManager.java
javax/swing/SwingUtilities.java
javax/swing/Timer.java
javax/swing/SwingWorker.java
javax/swing/SystemEventQueueUtilities.java
javax/swing/TimerQueue.java
javax/swing/TransferHandler.java
javax/swing/UIDefaults.java
javax/swing/UIManager.java
javax/swing/event/EventListenerList.java
javax/swing/colorchooser/DefaultHSBChooserPanel.java
javax/swing/colorchooser/SyntheticImage.java
javax/swing/plaf/basic/BasicDirectoryModel.java
javax/swing/plaf/basic/BasicLookAndFeel.java
javax/swing/plaf/basic/BasicPopupMenuUI.java
javax/swing/plaf/basic/BasicRadioButtonUI.java
javax/swing/plaf/basic/BasicSliderUI.java
javax/swing/plaf/basic/BasicTextUI.java
javax/swing/plaf/metal/MetalLookAndFeel.java
javax/swing/plaf/metal/MetalRadioButtonUI.java
javax/swing/plaf/metal/MetalToolBarUI.java
javax/swing/plaf/metal/MetalUtils.java
javax/swing/plaf/synth/ColorType.java
javax/swing/plaf/synth/DefaultSynthStyleFactory.java
javax/swing/plaf/synth/ImagePainter.java
javax/swing/plaf/synth/Region.java
javax/swing/plaf/synth/SynthStyle.java
javax/swing/plaf/synth/SynthContext.java
javax/swing/plaf/synth/SynthGraphicsUtils.java
javax/swing/plaf/synth/SynthLookAndFeel.java
javax/swing/table/TableColumn.java
javax/swing/text/AbstractDocument.java
javax/swing/text/AsyncBoxView.java
javax/swing/text/DefaultCaret.java
javax/swing/text/DefaultHighlighter.java
javax/swing/text/DefaultStyledDocument.java
javax/swing/text/ElementIterator.java
javax/swing/text/JTextComponent.java
javax/swing/text/LayoutQueue.java
javax/swing/text/SegmentCache.java
javax/swing/text/StringContent.java
javax/swing/text/StyleContext.java
javax/swing/text/#AbstractDocument.java#
javax/swing/text/html/CSS.java
javax/swing/text/html/ImageView.java
javax/swing/text/html/MuxingAttributeSet.java
javax/swing/text/html/StyleSheet.java
javax/swing/text/html/parser/Parser.java
javax/swing/text/html/parser/ParserDelegator.java
javax/swing/tree/DefaultTreeSelectionModel.java
javax/swing/undo/UndoManager.java
javax/swing/undo/UndoableEditSupport.java

some of these classes use synchronized internal to a method, others use them in the method declaration. Given that the keyword is an implementation detail, it should not be a backwards incompatibility to remove their usage.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
read the source, its used

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
methods without the synchronization keyword used.
ACTUAL -
synchronization used

REPRODUCIBILITY :
This bug can be reproduced always.