Arquivo Discussoes de Programacao e Tecnologia 
tá faltando alguma a lib org/apache/commons/collections/ArrayStack.
Veja em que jar ela está e joga o jar na lib do Tomcat
[TOMCAT_HOME]/server/lib/
Pode não ser só isso, mas em primeira instância pela exception parece ser:
java.lang.NoClassDefFoundError: org/apache/commons/collections/ArrayStack
abraço
Pessoal estou com esse erro quando dou um start no TomCat, alguem poderia me ajudar?
SEVERE: Exception sending context initialized event to listener instance of class com.sun.faces.config.ConfigureListener
java.lang.NoClassDefFoundError: org/apache/commons/collections/ArrayStack
at org.apache.commons.digester.Digester.(Digester.java:185)
at com.sun.faces.config.ConfigureListener.digester(ConfigureListener.java:964)
at com.sun.faces.config.ConfigureListener.contextInitialized(ConfigureListener.java:270)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3729)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4183)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:759)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:739)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:524)
at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:904)
at org.apache.catalina.startup.HostConfig.deployDirectories(HostConfig.java:867)
at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:474)
at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1112)
at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:310)
at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:119)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1021)
at org.apache.catalina.core.StandardHost.start(StandardHost.java:718)
at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1013)
at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:442)
at org.apache.catalina.core.StandardService.start(StandardService.java:450)
at org.apache.catalina.core.StandardServer.start(StandardServer.java:709)
at org.apache.catalina.startup.Catalina.start(Catalina.java:551)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:275)
at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:413)