Running AdvOperatorsExpertBenchmark benchmarks from our kie-benchmarks suite with parallel agenda option, the benchmark either cycles itself or throws NPE [1]. I created a "reproducer" here[2]. Be aware that this is failing nondeterministically, so with high probability it is a nasty race condition.

I tried to debug this a bit and this is the output of some variables that is available on this line [3]:
Returned fire count: 0
Head: null
Group: AgendaGroup 'MAIN'
Group is auto deactivate: true
Limit reached: false
IsInternalFire: false
IsFiring: true

[1] http://pastebin.com/sjLjQwgX
[2] https://github.com/kiegroup/kie-benchmarks/pull/8/files
[3] https://github.com/kiegroup/drools/blob/3038a1b9287978069bcb2765dc684f69683465a4/drools-core/src/main/java/org/drools/core/common/DefaultAgenda.java#L1084