The following query:

SELECT * FROM t1 WHERE f1 < CURDATE();

returns different results in 5.1-micro depending on the access method. FORCE KEY returns no rows while IGNORE KEY returns 2 (that is, all) rows in the table. In mysql-5.1 , both access methods return no rows.

Test case:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (f1 time , f2 varchar(5), KEY(f1));
INSERT INTO t1 VALUES ('00:20:01','a'),('00:20:03','b');
SELECT * FROM t1 FORCE KEY (f1) WHERE f1 < CURDATE();
SELECT * FROM t1 IGNORE KEY (f1) WHERE f1 < CURDATE();

In either version, for either access method, no warning is given even though we are comparing a DATE function to a TIME column.