title: How to fix "A signal was attempted before wait() on ConcurrencyManager" on EclipseLink?
body: <p>I have an JSF application running on a Tomcat server that users MySQL as database.
  The application runs normally until EclipseLink suddenlly throws a <code>ConcurrencyException</code>.
  I just can't figure out where the problem resides. Can somebody give me any clue?
  Exception showed below:</p>
  <pre><code>UnitOfWork(1713407022)--Exception [EclipseLink-2004] (Eclipse Persistence Services - 2.5.0.v20130507-3faac2b): org.eclipse.persistence.exceptions.ConcurrencyException
  Exception Description: A signal was attempted before wait() on ConcurrencyManager. This normally means that an attempt was made to 
  commit or rollback a transaction before it was started, or to rollback a transaction twice.
  </code></pre>