The overlap guarantee is strictly R + W > N. Not >=.

| N | R | W | R+W | overlaps | write survives | read survives | | --- | --- | --- | --- | --- | --- | --- | | 3 | 1 | 1 | 2 | no | 2 | 2 | | 3 | 2 | 2 | 4 | yes | 1 | 1 | | 3 | 1 | 3 | 4 | yes | 0 | 2 | | 3 | 3 | 1 | 4 | yes | 2 | 0 | | 5 | 2 | 3 | 5 | no | 2 | 3 | | 5 | 3 | 3 | 6 | yes | 2 | 2 |

Row five is the configuration people ship believing it is safe. R+W equals N, so a read quorum of two nodes can be entirely disjoint from the three that took the write. It returns stale data, with no error and no way for the caller to tell.