mysql testings> SELECT id, unread FROM messages_testing_small WHERE receiver=60505 ORDER BY id DESC limit 0,17;
+---------+--------+
| id      | unread |
+---------+--------+
| 3144053 | no     |
| 3143147 | no     |
| 3142989 | no     |
| 3141997 | no     |
| 3141945 | no     |
| 3141795 | no     |
| 3141053 | no     |
| 3141035 | no     |
| 3140863 | no     |
| 3140629 | no     |
| 3140523 | no     |
| 3140521 | no     |
| 3140517 | no     |
| 3140501 | no     |
| 3140481 | no     |
| 3140455 | no     |
| 3140331 | no     |
+---------+--------+
17 rows in set (0.00 sec)

mysql testings> SELECT id, unread FROM messages_testing_small WHERE receiver=60505 ORDER BY id DESC limit 0,18;
+---------+--------+
| id      | unread |
+---------+--------+
| 3144053 | no     |
| 3143803 | yes    |
| 3143147 | no     |
| 3142989 | no     |
| 3142237 | yes    |
| 3142171 | yes    |
| 3142167 | yes    |
| 3141997 | no     |
| 3141945 | no     |
| 3141925 | yes    |
| 3141889 | yes    |
| 3141863 | yes    |
| 3141795 | no     |
| 3141053 | no     |
| 3141035 | no     |
| 3140939 | yes    |
| 3140863 | no     |
| 3140629 | no     |
+---------+--------+
18 rows in set (0.00 sec)

Only limit param was changed between these two q.