When 2 rules share the same segment in sequential mode as in the following example:

rule R1 when
    $s : String()
    $m : Message()
    $i : Integer( this < $s.length )
then
    modify($m) { setMessage($s) };
end

rule R2 when
    $s : String()
    $m : Message()
    $i : Integer( this > $s.length )
then
end

the following NPE can be thrown:

java.lang.NullPointerException
        at org.drools.core.reteoo.BetaNode.getLeftIterator(BetaNode.java:418)
        at org.drools.core.phreak.PhreakJoinNode.doRightUpdates(PhreakJoinNode.java:287)
        at org.drools.core.phreak.PhreakJoinNode.doNode(PhreakJoinNode.java:48)
        at org.drools.core.phreak.RuleNetworkEvaluator.switchOnDoBetaNode(RuleNetworkEvaluator.java:547)
        at org.drools.core.phreak.RuleNetworkEvaluator.evalBetaNode(RuleNetworkEvaluator.java:533)
        at org.drools.core.phreak.RuleNetworkEvaluator.innerEval(RuleNetworkEvaluator.java:334)
        at org.drools.core.phreak.RuleNetworkEvaluator.outerEval(RuleNetworkEvaluator.java:161)
        at org.drools.core.phreak.RuleNetworkEvaluator.evaluateNetwork(RuleNetworkEvaluator.java:116)
        at org.drools.core.phreak.RuleExecutor.evaluateNetwork(RuleExecutor.java:87)
        at org.drools.core.common.DefaultAgenda.evaluateEagerList(DefaultAgenda.java:999)
        at org.drools.core.common.DefaultAgenda.fireNextItem(DefaultAgenda.java:954)
        at org.drools.core.common.DefaultAgenda.fireAllRules(DefaultAgenda.java:1243)
        at org.drools.core.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:950)
        at org.drools.core.common.AbstractWorkingMemory.fireAllRules(AbstractWorkingMemory.java:924)
        at org.drools.core.impl.StatefulKnowledgeSessionImpl.fireAllRules(StatefulKnowledgeSessionImpl.java:256)
        at org.drools.core.impl.StatelessKnowledgeSessionImpl.execute(StatelessKnowledgeSessionImpl.java:364)