Small app containing an extension (residing in gf/lib) and deployed war.
when server starts up (or the app is being deployed):

WARNING: Exception while dispatching an event
java.util.ConcurrentModificationException
at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
at java.util.HashMap$ValueIterator.next(HashMap.java:822)
at org.jboss.weld.bootstrap.WeldBootstrap.deployBeans(WeldBootstrap.java:370)
at org.glassfish.weld.WeldDeployer.event(WeldDeployer.java:151)
at org.glassfish.kernel.event.EventsImpl.send(EventsImpl.java:88)
at org.glassfish.internal.data.ApplicationInfo.load(ApplicationInfo.java:224)
at com.sun.enterprise.v3.server.ApplicationLifecycle.deploy(ApplicationLifecycle.java:322)
at com.sun.enterprise.v3.server.ApplicationLifecycle.deploy(ApplicationLifecycle.java:169)
at org.glassfish.deployment.admin.DeployCommand.execute(DeployCommand.java:272)
at com.sun.enterprise.v3.admin.CommandRunnerImpl$1.execute(CommandRunnerImpl.java:310)
at com.sun.enterprise.v3.admin.CommandRunnerImpl.doCommand(CommandRunnerImpl.java:320)
at com.sun.enterprise.v3.admin.CommandRunnerImpl.doCommand(CommandRunnerImpl.java:1176)
at com.sun.enterprise.v3.admin.CommandRunnerImpl.access$900(CommandRunnerImpl.java:83)
at com.sun.enterprise.v3.admin.CommandRunnerImpl$ExecutionContext.execute(CommandRunnerImpl.java:1235)
at org.glassfish.deployment.autodeploy.AutoOperation.run(AutoOperation.java:141)
at org.glassfish.deployment.autodeploy.AutoDeployer.deploy(AutoDeployer.java:573)
at org.glassfish.deployment.autodeploy.AutoDeployer.deployAll(AutoDeployer.java:459)
at org.glassfish.deployment.autodeploy.AutoDeployer.run(AutoDeployer.java:391)
at org.glassfish.deployment.autodeploy.AutoDeployer.run(AutoDeployer.java:376)
at org.glassfish.deployment.autodeploy.AutoDeployService$1.run(AutoDeployService.java:195)
at java.util.TimerThread.mainLoop(Timer.java:512)
at java.util.TimerThread.run(Timer.java:462)