Backend & systems·Consistency and replication
the network partitioned, both sides decided they were the leader, and both accepted writes to the same data.
Split brain
Two nodes simultaneously believing they hold exclusive leadership, usually after a partition or a slow heartbeat. The result is divergent state that has to be reconciled or discarded by hand. Preventing it is what majority quorums, fencing tokens and STONITH exist for; detecting it after the fact is expensive and manual.