If G1 needs to use the ParGCRareEvent_lock during GC, there will be a failure wrt to the rank of these two locks:

ParGCRareEvent_lock is of safepoint rank, and Threads_lock safepoint-1.

I.e. any (legitimate) use of ParGCRareEvent_lock during safepoint/gc will trigger the rank checking.

This rank issue occurs if verification finds any issue with eg. remembered sets, or references to dead objects etc. with -XX:-GCParallelVerificationEnabled (the default), then the VM thread which holds Threads_lock will fail.