We have seen nodes in Cassandra (3.0.11) ring gets into split-brain somehow. We don't know exact reproducible steps but here is our observation:
Let's assume we have 5 node cluster n1,n2,n3,n4,n5. In this bug when do nodetool status on each node then each one has different view of DN node
n1 sees n3 as DN and other nodes are UN
n3 sees n4 as DN and other nodes are UN
n4 sees n5 as DN and other nodes are UN and so on...
One thing we have observed is once n/w link is broken and restored then sometimes nodes go into this split-brain mode but we still don't have exact reproducible steps.
Please let us know if I am missing anything specific here.