If you set a SET column to an unsupported value, the value is ignored and a warning is issued : Set « Data Types « MySQL Tutorial

1. MySQL Tutorial
2. Data Types
3. Set

mysql>
mysql> CREATE TABLE myset (col SET('a', 'b', 'c', 'd'));
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO myset (col) VALUES ('a,d'),
    ->                                ('d,a'),
    ->                                ('a,d,a'),
    ->                                ('a,d,d'),
    ->                                ('d,a,d');
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql>
mysql> SELECT col FROM myset;
+------+
| col  |
+------+
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
+------+
5 rows in set (0.00 sec)

mysql>
mysql> INSERT INTO myset (col) VALUES ('a,d,d,s');
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql>
mysql> SHOW WARNINGS;
+---------+------+------------------------------------------+
| Level   | Code | Message                                  |
+---------+------+------------------------------------------+
| Warning | 1265 | Data truncated for column 'col' at row 1 |
+---------+------+------------------------------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT col FROM myset;
+------+
| col  |
+------+
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
| a,d  |
+------+
6 rows in set (0.00 sec)

mysql>
mysql>
mysql> drop table myset;
Query OK, 0 rows affected (0.00 sec)

mysql>