From time to time I see this error in server.log:
13:16:30,952 ERROR stderr#write Exception in thread "default task-36" java.lang.IllegalStateException: JBAS016071: Singleton not set for ModuleClassLoader for Module "org.wildfly.extension.io:main" from local module loader @782678 (finder: local module finder @25b780 (roots: /mbi/tools/jboss/8.0/modules,/mbi/tools/jboss/8.0/modules/system/layers/base)). This means that you are trying to access a weld deployment with a Thread Context ClassLoader that is not associated with the deployment.
13:16:30,953 ERROR stderr#write at org.jboss.as.weld.services.ModuleGroupSingletonProvider$TCCLSingleton.get(ModuleGroupSingletonProvider.java:75)
13:16:30,954 ERROR stderr#write at org.jboss.weld.Container.instance(Container.java:54)
13:16:30,955 ERROR stderr#write at org.jboss.weld.context.http.HttpSessionContextImpl.getConversationContext(HttpSessionContextImpl.java:63)
13:16:30,956 ERROR stderr#write at org.jboss.weld.context.http.HttpSessionContextImpl.destroy(HttpSessionContextImpl.java:39)
13:16:30,957 ERROR stderr#write at org.jboss.weld.servlet.WeldListener.sessionDestroyed(WeldListener.java:128)
13:16:30,957 ERROR stderr#write at io.undertow.servlet.core.ApplicationListeners.sessionDestroyed(ApplicationListeners.java:215)
13:16:30,958 ERROR stderr#write at io.undertow.servlet.core.SessionListenerBridge.sessionDestroyed(SessionListenerBridge.java:38)
13:16:30,959 ERROR stderr#write at io.undertow.server.session.SessionListeners.sessionDestroyed(SessionListeners.java:38)
13:16:30,960 ERROR stderr#write at io.undertow.server.session.InMemorySessionManager$SessionImpl.invalidate(InMemorySessionManager.java:276)
13:16:30,961 ERROR stderr#write at io.undertow.server.session.InMemorySessionManager$SessionImpl$1$1.run(InMemorySessionManager.java:139)
13:16:30,961 ERROR stderr#write at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
13:16:30,962 ERROR stderr#write at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
13:16:30,963 ERROR stderr#write at java.lang.Thread.run(Thread.java:722)