Bad Hash!
SELECT OLD_PASSWORD('aaaaaa') AS bad_hash UNION
-> SELECT OLD_PASSWORD('aaaaab') UNION
-> SELECT OLD_PASSWORD('aaaaac');
+------------------+
| bad_hash |
+------------------+
| 3af111f42d178bb8 |
| 3af113e92d178dad |
| 3af11d9e2d178f62 |
+------------------+]]>
Distribution of the hash key is bad and thus quite insecure