When clustering is enabled the following exception is seen on disconnecting from mininet. Try connecting and disconnecting couple of times with tree,4 topo

At first glance is seems to be an issue with FlowTopologyExporter.

2014-08-19 05:34:55 PDT [com.google.common.util.concurrent.ExecutionList] SEVERE com.google.common.util.concurrent.ExecutionList$RunnableExecutorPair execute RuntimeException while executing runnable com.google.common.util.concurrent.Futures$4@20c6e3a9 with executor com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService@618c7d0e
java.lang.IllegalStateException: Transaction was already submited.
at com.google.common.base.Preconditions.checkState(Preconditions.java:149)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMForwardedWriteTransaction.checkNotCommited(DOMForwardedWriteTransaction.java:157)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMForwardedWriteTransaction.checkNotReady(DOMForwardedWriteTransaction.java:148)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMForwardedWriteTransaction.delete(DOMForwardedWriteTransaction.java:96)
at org.opendaylight.controller.md.sal.dom.broker.impl.DOMForwardedWriteTransaction.delete(DOMForwardedWriteTransaction.java:53)
at org.opendaylight.controller.md.sal.binding.impl.AbstractWriteTransaction.doDelete(AbstractWriteTransaction.java:144)
at org.opendaylight.controller.md.sal.binding.impl.BindingDataWriteTransactionImpl.delete(BindingDataWriteTransactionImpl.java:73)
at org.opendaylight.md.controller.topology.manager.FlowCapableTopologyExporter$9$1.onSuccess(FlowCapableTopologyExporter.java:199)
at org.opendaylight.md.controller.topology.manager.FlowCapableTopologyExporter$9$1.onSuccess(FlowCapableTopologyExporter.java:192)
at com.google.common.util.concurrent.Futures$4.run(Futures.java:1149)
at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:293)
at com.google.common.util.concurrent.ExecutionList$RunnableExecutorPair.execute(ExecutionList.java:150)
at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:135)
at com.google.common.util.concurrent.AbstractFuture.set(AbstractFuture.java:185)
at com.google.common.util.concurrent.Futures$ChainingListenableFuture$1.run(Futures.java:873)
at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:293)
at com.google.common.util.concurrent.Futures$ImmediateFuture.addListener(Futures.java:99)
at com.google.common.util.concurrent.Futures$ChainingListenableFuture.run(Futures.java:866)
at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:293)
at com.google.common.util.concurrent.ExecutionList$RunnableExecutorPair.execute(ExecutionList.java:150)
at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:135)
at com.google.common.util.concurrent.ListenableFutureTask.done(ListenableFutureTask.java:91)
at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:380)
at java.util.concurrent.FutureTask.set(FutureTask.java:229)
at java.util.concurrent.FutureTask.run(FutureTask.java:270)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:744)