In the SQL below, the DELETE operation is a no-op.

<blockquote><verbatim>
CREATE TABLE t1(a INT UNIQUE, b INT);
INSERT INTO t1(a,b) VALUES('1','2');
SELECT * FROM t1 WHERE a='1' AND b='2';
DELETE FROM t1 WHERE a='1' AND b='2';
SELECT * FROM t1;
</verbatim></blockquote>

Even the SELECT on the third line outputs the row, and the DELETE has the
exact same WHERE clause, the DELETE does not delete the row.

Bisecting suggests that this problem was introduced by 
check-in [85c467041c9378ca] on 2016-01-30 for version 3.11.0.