If you use the deployment task supplied by jBPM in ant to deploy process definitions and you forget to take care of the deployment of the jar-file with the classes like decision handlers referenced in the process definition the deployment will be successful but you will run into trouble afterwards. You won't be able to view the list of available process definitions in the jbpm-console but instead an exception is thrown:

java.lang.ClassCastException: org.jbpm.jpdl.internal.model.JpdlProcessDefinition cannot be cast to java.util.List

Using deleteDeployment or deleteDeploymentCascade offered by the RepositoryService you won't be able to remove the deployments since you run into the same exception. So this actually breaks your jbpm-installation. Putting the jar into the lib-Folder of Tomcat and a restart seem to help but it is strange that missing classes break the process definition overview and that the deployments cannot be deleted.

The same problem occurs after the deployment of process definitions which reference mail templates. Suppose the following situation: You want to deploy a process definition which references a mail template in a mail task. This does not work unless you define the template in the configuration file used by the ant deployment task. But if you forget to also define them on the server you can't undeploy the process definition afterwards. Of course you could just go and add the mail template definition but what about typos etc.?