See http://drools-java-rules-engine.46999.n3.nabble.com/StatefulKnowledgeSession-startProcess-not-thread-safe-td1973203.html
and http://drools-java-rules-engine.46999.n3.nabble.com/Weird-Problem-WorkItemManager-seems-to-loose-WorkItems-td1986222.html

The documentation lets believe that the StatefulKnowledgeSession can be used in a multithreaded environment (https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.0-SNAPSHOT-docs-build/jbpm-docs/html/ch04.html#d0e1191).

But the classes org.drools.process.instance.impl.DefaultProcessInstanceManager and org.drools.process.instance.impl.DefaultWorkItemManager are not synchronized.
Consequences: process instance ids and workitem ids not unique.