Any Idea Why This Happens?

Running data flow operator demo
x was set to 23
x was set to 23
An exception occurred in the Actor thread Actor Thread 2
java.lang.IllegalStateException: A DataFlowVariable can only be assigned once.
	at groovyx.gpars.dataflow.DataFlowExpression.bind(DataFlowExpression.java:292)
	at groovyx.gpars.dataflow.DataFlowVariable.leftShift(DataFlowVariable.java:50)
	at groovyx.gpars.dataflow.DataFlowVariable$leftShift.call(Unknown Source)
	at groovyx.gpars.dataflow.operator.DataFlowOperator.bindOutput(DataFlowOperator.groovy:82)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:592)
	at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:88)
	at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:233)
	at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod(ClosureMetaClass.java:361)
	at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:886)
	at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.callCurrent(PogoMetaClassSite.java:66)
	at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callCurrent(AbstractCallSite.java:155)
	at demo.SimpleDataFlowOperatorDemo$_main_closure1.doCall(SimpleDataFlowOperatorDemo.groovy:18)