Most notably servers that invalidate sessions before they get passed to the session listener. (tomcat is one example)

This is the stack trace that normally appears in the logs:

2006-01-05 13:20:25 StandardManager[] Session event listener threw exception
java.lang.IllegalStateException: getAttribute: Session already invalidated
at org.apache.catalina.session.StandardSession.getAttribute(StandardSession.java:953)
at org.apache.catalina.session.StandardSessionFacade.getAttribute(StandardSessionFacade.java:171)
at com.atlassian.confluence.user.listeners.UserSessionExpiryListener.sessionDestroyed(UserSessionExpiryListener.java:28)
at org.apache.catalina.session.StandardSession.expire(StandardSession.java:658)
at org.apache.catalina.session.StandardSession.expire(StandardSession.java:607)
at org.apache.catalina.session.StandardManager.processExpires(StandardManager.java:793)
at org.apache.catalina.session.StandardManager.run(StandardManager.java:870)
at java.lang.Thread.run(Thread.java:534)