We have all been caught out with using SET and not realizing that the default GLOBAL Scope (since 5.0.2) does not change the current SESSION scope.

I was not aware until today that changing GLOBAL scope has some exceptions that also automatically affect SESSION scope.

What I expected with a change in the GLOBAL scope is no affect SESSION scope. For example.

mysql> SHOW GLOBAL VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 131072 |
+------------------+--------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 131072 |
+------------------+--------+
1 row in set (0.00 sec)

mysql> SET GLOBAL read_buffer_size=1024*256;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 262144 |
+------------------+--------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'read_buffer_size';
+------------------+--------+
| Variable_name    | Value  |
+------------------+--------+
| read_buffer_size | 131072 |
+------------------+--------+
1 row in set (0.00 sec)

However I was no prepared for this when changing an important variable for transaction management.

mysql> SHOW GLOBAL VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

mysql> SET autocommit=0;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | OFF   |
+---------------+-------+
1 row in set (0.00 sec)

However even more perplexing was the following message:

mysql> SET GLOBAL autocommit=0;
ERROR 1228 (HY000): Variable 'autocommit' is a SESSION variable and can't be used with SET GLOBAL

So this is another case were the definition of variables is not applicable in a GLOBAL level, yet the tools of the trade represent in some manner misleading information.
To prove my point, here is another new concurrent session started after the above.

mysql> SHOW GLOBAL VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.01 sec)

mysql> SHOW SESSION VARIABLES LIKE 'autocommit';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| autocommit    | ON    |
+---------------+-------+
1 row in set (0.00 sec)

The MySQL Documentation also had an incorrect specification with description regarding this scope, Bug #54215