Running any of the examples yields in the following exception:

Exception in thread "AWT-EventQueue-0" java.lang.IllegalAccessError: tried
to access method
antlr.CommonHiddenStreamToken.setHiddenBefore(Lantlr/CommonHiddenStreamToken;)V
from class antlr.TokenStreamCopyingHiddenTokenFilter
at
antlr.TokenStreamCopyingHiddenTokenFilter.nextToken(TokenStreamCopyingHiddenTokenFilter.java:162)
at antlr.TokenBuffer.fill(TokenBuffer.java:69)
at antlr.TokenBuffer.LA(TokenBuffer.java:80)
at antlr.LLkParser.LA(LLkParser.java:52)
at processing.app.preproc.PdeRecognizer.pdeProgram(PdeRecognizer.java:65)
at processing.app.preproc.PdePreprocessor.write(PdePreprocessor.java:318)
at processing.app.Sketch.build(Sketch.java:1458)
at processing.app.Sketch.handleCompile(Sketch.java:1326)
at processing.app.Editor.handleRun(Editor.java:1140)
at processing.app.EditorToolbar.mousePressed(EditorToolbar.java:321)
at java.awt.Component.processMouseEvent(Component.java:5599)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3135)
at java.awt.Component.processEvent(Component.java:5367)
at java.awt.Container.processEvent(Container.java:2010)
at java.awt.Component.dispatchEventImpl(Component.java:4068)
at java.awt.Container.dispatchEventImpl(Container.java:2068)
at java.awt.Component.dispatchEvent(Component.java:3903)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4256)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3933)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3866)
at java.awt.Container.dispatchEventImpl(Container.java:2054)
at java.awt.Window.dispatchEventImpl(Window.java:1791)
at java.awt.Component.dispatchEvent(Component.java:3903)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:463)
at
java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchThread.java:269)
at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:190)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:176)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:110)

System: os x 10.5.2
Java: os x's native java v1.5 (same happens with former 1.4.2, or the 1.6
patch from apple)
Processing: 0137, 0135, 0125

Has anyone come across this? Is it a bug or just something wrong with my
environment?

Thanks for any hints!

look for another copy of antlr.jar on your system, it's causing a conflict.