I just got this when I posted in the "favorite sayings" thread:
HTTP Status 500 -

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type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

net.sf.hibernate.AssertionFailure: null id in entry (don't flush the Session after an exception occurs)
net.sf.hibernate.impl.SessionImpl.checkId(SessionImpl.java:2641)
net.sf.hibernate.impl.SessionImpl.flushEntity(SessionImpl.java:2465)
net.sf.hibernate.impl.SessionImpl.flushEntities(SessionImpl.java:2458)
net.sf.hibernate.impl.SessionImpl.flushEverything(SessionImpl.java:2260)
net.sf.hibernate.impl.SessionImpl.autoFlushIfRequired(SessionImpl.java:1805)
net.sf.hibernate.impl.SessionImpl.getQueries(SessionImpl.java:1568)
net.sf.hibernate.impl.SessionImpl.find(SessionImpl.java:1533)
net.sf.hibernate.impl.QueryImpl.list(QueryImpl.java:39)
com.ezabel.services.DataServiceHibernate.getBanByIP(DataServiceHibernate.java:96)
com.ezabel.jsp.AutoLoginFilter.doFilter(AutoLoginFilter.java:39)

note The full stack trace of the root cause is available in the Apache Tomcat/5.5.7 logs.

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Apache Tomcat/5.5.7