it inserts '' when you try to insert an illegal value, rather than returning an error

mysql enum sux

mysql> create database foo;
Query OK, 1 row affected (0.00 sec)

mysql> use foo;
Database changed
mysql> create table blah (e ENUM('True', 'False') NOT NULL);
Query OK, 0 rows affected (0.10 sec)

mysql> INSERT INTO blah VALUES ('Flase');
Query OK, 1 row affected, 1 warning (0.04 sec)

mysql> select * from blah;
+---+
| e |
+---+
|   |
+---+
1 row in set (0.00 sec)