While testing the changes for 7119908 I ran the GC test suite with -XX:ParallelGCThreads==0 and got several failures - specifically in GCBasher and dacapo2006.
The bulk of these failures were SEGVs due to accessing the FlexibleWorkGang (which does not exist when whne PGCT==0), others were asserts.
There is an RFE to remove the PGCT==0 specific code but until then...
Various SEGVs and asserts when PGCT==0. Most are the result of accessing the worker gang (which is only created when PGCT!=0) outside of a suitable guard.