I'm trying (unsuccessfully for the time being) to create a test case that will reproduce it.
It happens randomly with 0.9.30 when running a regression testing with TestNG + Zookeeper + Own code.
Apparently it does not happen with 0.9.29.

java.lang.IllegalMonitorStateException
at java.util.concurrent.locks.ReentrantReadWriteLock$Sync.tryReleaseShared(Unknown Source)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.releaseShared(Unknown Source)
at java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.unlock(Unknown Source)
at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:68)
at ch.qos.logback.classic.Logger.appendLoopOnAppenders(Logger.java:285)
at ch.qos.logback.classic.Logger.callAppenders(Logger.java:272)
at ch.qos.logback.classic.Logger.buildLoggingEventAndAppend(Logger.java:473)
at ch.qos.logback.classic.Logger.filterAndLog_0_Or3Plus(Logger.java:427)
at ch.qos.logback.classic.Logger.info(Logger.java:631)
at org.apache.zookeeper.server.ZooKeeperServer.shutdown(ZooKeeperServer.java:420)
at org.apache.zookeeper.server.quorum.Leader.shutdown(Leader.java:443)
at org.apache.zookeeper.server.quorum.QuorumPeer.shutdown(QuorumPeer.java:766)
at org.apache.zookeeper.test.QuorumUtil.shutdown(QuorumUtil.java:224)
at org.apache.zookeeper.test.QuorumUtil.shutdownAll(QuorumUtil.java:211)
at org.apache.zookeeper.test.QuorumUtil.tearDown(QuorumUtil.java:255)
at eswf.zk2.agents.ZkEphemeralInfoPublisherTest.tearDown(ZkEphemeralInfoPublisherTest.java:55)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:80)
at org.testng.internal.Invoker.invokeConfigurationMethod(Invoker.java:543)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:212)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:772)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:883)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1208)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:127)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:111)
at org.testng.TestRunner.privateRun(TestRunner.java:753)
at org.testng.TestRunner.run(TestRunner.java:613)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:335)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:330)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:292)
at org.testng.SuiteRunner.run(SuiteRunner.java:241)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1169)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1094)
at org.testng.TestNG.run(TestNG.java:1006)
at org.testng.remote.RemoteTestNG.run(RemoteTestNG.java:107)
at org.testng.remote.RemoteTestNG.initAndRun(RemoteTestNG.java:199)
at org.testng.remote.RemoteTestNG.main(RemoteTestNG.java:170)