A misconfigured web.xml throws a null pointer exception:

<welcome-file-list>
<welcome-file>secure/main.html</welcome-file> <!-- missing a slash before 'secure', ie: /secure/main.html -->
</welcome-file-list>

throws exception:

10:20:00,977 ERROR [CoyoteAdapter] An exception or error occurred in the container during the request processing
java.lang.NullPointerException
at org.jboss.security.plugins.javaee.WebAuthorizationHelper.hasUserDataPermission(WebAuthorizationHelper.java:175)
at org.jboss.web.tomcat.security.JBossWebRealm.hasUserDataPermission(JBossWebRealm.java:636)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:461)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:91)
at org.jboss.web.tomcat.security.SecurityContextEstablishmentValve.invoke(SecurityContextEstablishmentValve.java:92)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:325)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:828)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:601)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)

while not crucial to the development of jboss, I speak from experience when I say that it can take a long time to find the configuration error based upon the exception thrown...