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After enabling the warning when mutex goes from read to write access one or two of its tests seem to expose this behaviour. If the tests are wrong then get rid of them, if they are ok and should show the warning, register your own ErrorManager and verify that the warning is really shown. An example how to register ErrorManager is in http://www.netbeans.org/source/browse/openide/test/unit/src/org/openide/util/actions/AsynchronousTest.java?rev=1.1&content-type=text/x-cvsweb-markup
The test tested wrong behaviour. For now, I've commented out the test, because the Mutex should actually throw the exception on upgrade, not only show the warning.