That's a syntax error.

'''mysql> DELETE FROM waffle_flag_mkt_users WHERE flag_id = (SELECT id FROM waffle_flag_mkt WHERE name = 'override-app-purchase' INTO @flag_id);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INTO @flag_id)' at line 1'''

I was trying to spot the storing in a variable syntax in the MySQL sub query docs http://dev.mysql.com/doc/refman/5.0/en/subqueries.html but couldn't. Any pointers?