Sqlyog 6.0.6 Beta 1 on Windows XP SP2, MySQL 5.0.45 can crashes easily simply by executing the following sql queries
create table a (a int, b int);
create table b (a int, b int);
insert into a values (1,1),(2,1),(3,1);
insert into b values (1,1), (3,1);
update a left join b on a.a=b.a set a.b=2, b.b=2 where (a.b=1 and b.b=1) or b.a is NULL;
It can be reproducible in Sqlyog 6.0.5 as well.