Occasionally seeing this type of exception:

02:02:27,883 |-ERROR in com.electriccloud.log.AsyncAppender[ASYNC_FILE] - Appender [ASYNC_FILE] failed to append. java.util.ConcurrentModificationExceptionjava.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$EntryIterator.next(HashMap.java:834)
at java.util.HashMap$EntryIterator.next(HashMap.java:832)
at java.util.HashMap.putAllForCreate(HashMap.java:435)
at java.util.HashMap.<init>(HashMap.java:225)
at ch.qos.logback.classic.spi.LoggingEvent.prepareForDeferredProcessing(LoggingEvent.java:213)
at com.electriccloud.log.AsyncAppender.append(AsyncAppender.java:132)
at com.electriccloud.log.AsyncAppender.append(AsyncAppender.java:36)
at ch.qos.logback.core.AppenderBase.doAppend(AppenderBase.java:85)
at ch.qos.logback.core.spi.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:64)
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.debug(Logger.java:534)
at com.electriccloud.log.Slf4jLog.debug(Slf4jLog.java:66)
at com.electriccloud.log.Slf4jLog.log(Slf4jLog.java:202)
at com.electriccloud.util.timer.Timer.logElapsed(Timer.java:324)
at com.electriccloud.operations.OperationTimingAspect.logOperationPerformance(OperationTimingAspect.java:107)
at sun.reflect.GeneratedMethodAccessor124.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethodWithGivenArgs(AbstractAspectJAdvice.java:627)
at org.springframework.aop.aspectj.AbstractAspectJAdvice.invokeAdviceMethod(AbstractAspectJAdvice.java:616)
at org.springframework.aop.aspectj.AspectJAroundAdvice.invoke(AspectJAroundAdvice.java:64)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:160)
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:171)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:204)
at $Proxy23.perform(Unknown Source)
at com.electriccloud.operations.OperationInvoker.invoke(OperationInvoker.java:273)
at com.electriccloud.operations.OperationCaller.call(OperationCaller.java:127)
at com.electriccloud.operations.OperationCaller.call(OperationCaller.java:42)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
at com.electriccloud.util.CommanderThread.run(CommanderThread.java:105)