As of JDK 7 b38.
Consider the following sequence, plainName is NOT a pattern:

EventClient.subscribe(plainName, listener, null, null);
MBeanServer.unregister(plainName);
EventClient.unsubscribe(plainName, listener);

==> you get a ListenerNotFoundException, see the stack below.

[1] the javadoc doesn't say ListenerNotFoundException is thrown in such a case. It's a disconnect between implementation and spec.
Current javadoc says:
Throws:
    ListenerNotFoundException - The given listener was not subscribed to the given name.

[2] If in place of a plain name you use a pattern you don't get any exception. The behavior isn't symmetric, this isn't intuitive.

javax.management.ListenerNotFoundException: The listener is not found.
at javax.management.event.EventClientDelegate$ClientInfo.removeListenerInfo(EventClientDelegate.java:570)
at javax.management.event.EventClientDelegate.removeListenerOrSubscriber(EventClientDelegate.java:416)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:623)
at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:112)
at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:46)
at com.sun.jmx.mbeanserver.MBeanIntrospector.invokeM(MBeanIntrospector.java:292)
at com.sun.jmx.mbeanserver.PerInterface.invoke(PerInterface.java:138)
at com.sun.jmx.mbeanserver.MBeanSupport.invoke(MBeanSupport.java:252)
at javax.management.StandardMBean.invoke(StandardMBean.java:648)
at javax.management.namespace.MBeanServerSupport.invoke(MBeanServerSupport.java:702)
at com.sun.jmx.interceptor.SingleMBeanForwarder.invoke(SingleMBeanForwarder.java:279)
at sun.reflect.GeneratedMethodAccessor6.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:623)
at javax.management.ClientContext$ContextInvocationHandler.invoke(ClientContext.java:1023)
at javax.management.ClientContext$ContextInvocationHandler.invoke(ClientContext.java:969)
at $Proxy1.invoke(Unknown Source)
at javax.management.remote.IdentityMBeanServerForwarder.invoke(IdentityMBeanServerForwarder.java:161)
at com.sun.jmx.interceptor.SingleMBeanForwarder.invoke(SingleMBeanForwarder.java:281)
at javax.management.remote.IdentityMBeanServerForwarder.invoke(IdentityMBeanServerForwarder.java:161)
at javax.management.remote.rmi.RMIConnectionImpl.doOperation(RMIConnectionImpl.java:1754)
at javax.management.remote.rmi.RMIConnectionImpl.access$1000(RMIConnectionImpl.java:99)
at javax.management.remote.rmi.RMIConnectionImpl$PrivilegedOperation.run(RMIConnectionImpl.java:1599)
at javax.management.remote.rmi.RMIConnectionImpl.doPrivilegedOperation(RMIConnectionImpl.java:1687)
at javax.management.remote.rmi.RMIConnectionImpl.invoke(RMIConnectionImpl.java:813)
at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:623)
at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:322)
at sun.rmi.transport.Transport$1.run(Transport.java:177)
at sun.rmi.transport.Transport$1.run(Transport.java:174)
at java.security.AccessController.doPrivileged(Native Method)
at sun.rmi.transport.Transport.serviceCall(Transport.java:173)
at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:553)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:808)
at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:667)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:713)