See this example project: https://github.com/DuncanDoyle/DroolsReteViewer , which is based on the Drools Eclipse sample project.

The 'src/main/resources/rules/Sample.drl' is a valid DRL file, but it cannot be opened in the Rete Tree because of:

• the 'timer (int: 10s)' definition in rule "GoodBye-Timer". This gives the error:
  Rete Tree Build Error!
  Reason:
  org.drools.core.RuntimeDroolsException:
  java.lang.reflect.InvocationTargetException: [Rete(0)]

• the '||' operator in the rule "Hello World-Or". This gives the error:
  Rete Tree Build Error!
  Reason:
  java.lang.NullPointerException: null

This forces us to change our rule definition files to be able to inspect the Rete Tree (which we require to analyse the tree for possible optimizations).