The following is logged every 3-5 seconds in our installation. We mostly use Swarm slaves, about 100 of them.

Unreliable Slave plugin 1.2, Jenkins 1.646 on Centos 6.5, OpenJDK 1.8

Feb 23, 2016 8:49:11 AM WARNING hudson.model.listeners.RunListener report
RunListener failed
java.lang.NullPointerException
at org.jenkinsci.plugins.detection.unreliable.slave.BuildStatisticListener.onCompleted(BuildStatisticListener.java:54)
at hudson.model.listeners.RunListener.fireCompleted(RunListener.java:201)
at hudson.maven.MavenBuild$ProxyImpl2.end(MavenBuild.java:602)
at sun.reflect.GeneratedMethodAccessor624.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at hudson.model.Executor$2.call(Executor.java:937)
at hudson.util.InterceptingProxy$1.invoke(InterceptingProxy.java:23)
at com.sun.proxy.$Proxy65.end(Unknown Source)
at sun.reflect.GeneratedMethodAccessor21290.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:608)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:583)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:542)
at hudson.remoting.UserRequest.perform(UserRequest.java:120)
at hudson.remoting.UserRequest.perform(UserRequest.java:48)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at org.jenkinsci.remoting.CallableDecorator.call(CallableDecorator.java:18)
at hudson.remoting.CallableDecoratorList$1.call(CallableDecoratorList.java:21)
at jenkins.util.ContextResettingExecutorService$2.call(ContextResettingExecutorService.java:46)
at java.util.concurrent.FutureTask.run(FutureTask.java:262)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:745)