The bug was originally filed by a user at bugs.mysql.com. It's reproducible on MySQL 5.6 as well as MariaDB 5.3-10.0.

See the original description, including the test case, and the suggested patch in http://bugs.mysql.com/bug.php?id=68254 (I don't know if I may copy-paste it).

The major complaint is that if table t1 has rows

a b

----

1 0

2 1

3 1

4 1

then, while a query

SELECT a from t1 group by b

returns

1

2

The query

SELECT x,y FROM t2 WHERE y IN (SELECT a FROM t1 GROUP BY b)

returns rows with y IN (1,2,3,4).

I've set it to Minor for now, because the query with GROUP BY is indeterministic according to MySQL documentation, so I don't see that much value in using it as a subquery; but it's still a bug since the subquery is supposed to return 2 values, not 4, even it can be any of (1,2), (1,3) or (1,4).

MySQL has verified the bug, although I don't know if they're going to fix it, and how.