Same Arithmetic, Broken Freshness Diagram

Lost ACKs

Two ACKs lost: the coordinator reports v8 failed, three nodes hold it
A later read quorum returns the failed write: failure is not rollback
Retry without version fencing can fork the value; fence it

Stale replacement

n3 is rebuilt from an old snapshot and forgets the v8 it ACKed
The overlap is still {n3}, but the bridge carries v6, not v8
Same sets, same arithmetic, older value: the premise broke, not the sum

Membership change

A read quorum drawn from new members never met the old write set
w + r > n must be computed over one membership, not two
During rebalancing, old and new quorums must overlap each other