The following query:

SELECT ( SELECT MIN( t1.a ) FROM t1, t2 WHERE t2.a = t3.t1 ) FROM t3;

returns

even though the correct result should be

as there are no rows for which t2.a = t3.t1 is true

explain:

repeatable in maria-5.3, maria-5.2, mysql-5.5

test case:

CREATE TABLE t1 ( a int NOT NULL, PRIMARY KEY (a)) engine=myisam;
INSERT INTO t1 VALUES (1);

CREATE TABLE t2 ( a int NOT NULL ) engine=myisam;
INSERT INTO t2 VALUES (10);

CREATE TABLE t3 ( a int NOT NULL , t1 int) engine=myisam;
INSERT INTO t3 VALUES (19,1),(20,5);

SELECT ( SELECT MIN( t1.a ) FROM t1, t2 WHERE t2.a = t3.t1 ) FROM t3;