The following sequence of commands gives us an unexpected error message in MariaDB 5.5:

CREATE TABLE t1 (a int);
CREATE TABLE t2 (b int);
CREATE TABLE t3 (c int);
SELECT a FROM t1 UNION SELECT b FROM t2 JOIN (t3) ON ( t2.b = t3.c );

MariaDB [test]> SELECT a FROM t1 UNION SELECT b FROM t2 JOIN (t3) ON ( t2.b = t3.c );
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'ON ( t2.b = t3.c )' at line 1

If we exchange SELECTs in the above UNION the query will be accepted though:

MariaDB [test]> SELECT b FROM t2 JOIN (t3) ON ( t2.b = t3.c ) UNION SELECT a FROM t1;
Empty set (0.00 sec)

The versions MariaDB 5.2/5.3 are not affected, while mysql-5.5 is affected: see http://bugs.mysql.com/bug.php?id=54382