A very simple example "FailTest" triggers a rule only 15 times when it should trigger it 16 times.
The same example with almost any change to the class model or to the order in loading the classes or the input data or the version of java it works.
The Class FailTest contains only a main method calling the main method of the Class HelloWorldExample. Running directly the main method in the class HelloWorldExample does work.

Any change fixes the problem in the example but it moves the error to other part. We have set a test testing more than 2000 million cases of rules/inputs and no matter how we write the rules or the code, we end up finding a case that fails, making the software not usable, as it is not predictable were it is going to fail.
The case has been uner study for more than a year and we have tried AIX, Linux and XP with DROOLS 5.1, 5.2, 5.3 and the trial of Enterprise edition. The example provided has only been tested to fail for DROOLS 5.3.0 final and Enterprise edition for SLES11 SP1 with the SUN virtual machine (not the IBM one) and for XP but there is no combination for which DROOLS work for the 2000 million cases.

The example provided is a 7kb example self-contained ( attached file DROOLS_ERROR.zip )