Split Brain
A partition in which two halves of a cluster each believe they are authoritative, and both accept writes.
The classic failover accident: the primary is not dead, it is unreachable. The standby is promoted, the original recovers still believing it is primary, and now two nodes are accepting conflicting writes to what is supposed to be one dataset. When the partition heals, there is no correct merge.
The defences, in order of reliability: quorum so a minority partition refuses to serve; fencing tokens so the storage layer rejects writes from a deposed leader; and STONITH — having the promoting node forcibly power off the old one before taking over, which is crude and effective.
What does not work is a heartbeat timeout on its own. It cannot distinguish "dead" from "slow", and tuning it shorter trades split-brain risk for spurious failover risk.