Both hardy (5.0.51a) and karmic (5.0.75) don't allow the use of such a query:
mysql> SELECT * FROM (SELECT mu.User FROM mysql.user mu UNION SELECT mu.user FROM mysql.user mu ORDER BY mu.User) a;
ERROR 1054 (42S22): Unknown column 'mu.User' in 'order clause'
which seems to be right behavior.
Marking this bug Fixed Released in hardy and above.
Both hardy (5.0.51a) and karmic (5.0.75) don't allow the use of such a query:
mysql> SELECT * FROM (SELECT mu.User FROM mysql.user mu UNION SELECT mu.user FROM mysql.user mu ORDER BY mu.User) a;
ERROR 1054 (42S22): Unknown column 'mu.User' in 'order clause'
which seems to be right behavior.
Marking this bug Fixed Released in hardy and above.