To reproduce the problem:

DROP TABLE IF EXISTS t1;
CREATE TABLE `t1` (
`id` int(10) NOT NULL AUTO_INCREMENT,
`t1_date` datetime NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=cp1251;
INSERT INTO t1 (t1_date) VALUES (NOW());

Set character and collation to cp1251:

CREATE DATABASE `test_cp1251` DEFAULT CHARACTER SET cp1251 ;

Query:

SELECT date(t1_date) FROM t1 WHERE (CASE WHEN 1 THEN date(t1_date) ELSE null END >= '2013-12-01 00:00:00');

return the error:

ERROR 1267 (HY000): Illegal mix of collations (latin1_swedish_ci,NUMERIC) and (cp1251_general_ci,COERCIBLE) for operation '>='

If I return configuration to UTF8, all work fine:

SET NAMES utf8 COLLATE utf8_unicode_ci;
SELECT date(t1_date) FROM t1 WHERE (CASE WHEN 1 THEN date(t1_date) ELSE null END >= '2013-12-01 00:00:00');