Hi ricardo,
Like maciej pointed out, check for multiple instances of jbpm if you are deploying it on AS. Also to add to his post, if you use Jboss and use jbpm as service in your application, check how you bind and unbind ProcessEngine to JNDI (I'm not sure whether the supplied service jbpm-jboss5.jar is properly written). This can cause classcast exceptions on application hot deploy.