Split brain occurs when both nodes believe they are the primary. Both respond to requests, causing duplicate packets, conflicting state, and data corruption.
Causes include: network partition between nodes (heartbeats fail but both are up), misconfigured priorities, or clock drift.
Prevention techniques:
Quorum: Require majority vote before taking over. With nodes, must agree.
STONITH: "Shoot The Other Node In The Head." Force-reboot the other node before taking over.
Fencing: Physically isolate the old primary from shared resources.