In MySQL 5.5, leap second values are returned with a time part that ends with :59:59. This means that a function such as NOW() can return the same value for two or three consecutive seconds during the leap second. It remains true that literal temporal values having a time part that ends with :59:60 or :59:61 are considered invalid.

If it is necessary to search for TIMESTAMP values one second before the leap second, anomalous results may be obtained if you use a comparison with 'YYYY-MM-DD hh:mm:ss' values:

mysql> CREATE TABLE t1 (a INT, ts TIMESTAMP DEFAULT NOW(), PRIMARY KEY (ts));
Query OK, 0 rows affected (0.11 sec)

mysql> # Simulate NOW() = '2009-01-01 02:59:59'
mysql> SET timestamp = 1230768022;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 (a) VALUES (1);
Query OK, 1 row affected (0.07 sec)

mysql> # Simulate NOW() = '2009-01-01 02:59:60'
mysql> SET timestamp = 1230768023;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1 (a) VALUES (2);
Query OK, 1 row affected (0.02 sec)

mysql> SELECT * FROM t1;
+------+---------------------+
| a    | ts                  |
+------+---------------------+
|    1 | 2008-12-31 18:00:22 |
|    2 | 2008-12-31 18:00:23 |
+------+---------------------+
2 rows in set (0.02 sec)

mysql> SELECT * FROM t1 WHERE ts = '2009-01-01 02:59:59';
Empty set (0.03 sec)

To work around this, you can use a comparison based on the UTC value actually stored in column, which has the leap second correction applied:

mysql> SELECT * FROM t1 WHERE UNIX_TIMESTAMP(ts) = 1230768023;
+------+---------------------+
| a    | ts                  |
+------+---------------------+
|    2 | 2008-12-31 18:00:23 |
+------+---------------------+
1 row in set (0.02 sec)