With a wrong module.xml you Extension will be unable to start and show a stacktrace not unlike:
12:52:11,273 ERROR [org.jboss.msc] MSC-00001: Failed to start service jboss.as.server-controller: org.jboss.msc.service.StartException in service jboss.as.server-controller: Failed to start service
at org.jboss.msc.service.ServiceInstanceImpl$StartTask.run(ServiceInstanceImpl.java:1207)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [:1.6.0_22]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [:1.6.0_22]
at java.lang.Thread.run(Thread.java:662) [:1.6.0_22]
Caused by: java.util.ServiceConfigurationError: org.jboss.as.server.Extension: Provider org.jboss.as.ejb3.EJB3Extension not found
at java.util.ServiceLoader.fail(ServiceLoader.java:214) [:1.6.0_22]
at java.util.ServiceLoader.access$400(ServiceLoader.java:164) [:1.6.0_22]
at java.util.ServiceLoader$LazyIterator.next(ServiceLoader.java:350) [:1.6.0_22]
at java.util.ServiceLoader$1.next(ServiceLoader.java:421) [:1.6.0_22]
at org.jboss.as.model.ModelXmlParsers.parseExtensions(ModelXmlParsers.java:432)
at org.jboss.as.model.ModelXmlParsers.parseServerRootElement(ModelXmlParsers.java:180)
at org.jboss.as.model.ModelXmlParsers$3.readElement(ModelXmlParsers.java:108)
at org.jboss.as.model.ModelXmlParsers$3.readElement(ModelXmlParsers.java:106)
at org.jboss.staxmapper.XMLMapperImpl.processNested(XMLMapperImpl.java:91)
at org.jboss.staxmapper.XMLMapperImpl.parseDocument(XMLMapperImpl.java:59)
at org.jboss.as.server.mgmt.StandaloneServerConfigurationPersister.load(StandaloneServerConfigurationPersister.java:109)
at org.jboss.as.server.ServerControllerService.start(ServerControllerService.java:137)
at org.jboss.msc.service.ServiceInstanceImpl$StartTask.run(ServiceInstanceImpl.java:1163)
In this thread I'll write up the steps needed to debug such an issue.