ERROR 1396 (HY000): Operation CREATE USER failed for 'jack'@'localhost'

I seem to be unable to re-create a simple user I've deleted, even as root in MySQL. My case: user 'jack' existed before, but I deleted it from mysql.user in order to recreate it. I see no vestiges of this in that table. If I execute this command for some other, random username, say 'jimmy', it works fine (just as it originally did for 'jack'). What have I done to corrupt user 'jack' and how can I undo that corruption in order to re-create 'jack' as a valid user for this installation of MySQL? See example below. (Of course, originally, there was much time between the creation of 'jack' and his removal.) mysql> CREATE USER 'jack'@'localhost' IDENTIFIED BY 'test123'; Query OK, 0 rows affected (0.00 sec) mysql> select user,host from user; +------------------+-----------------+ | user | host | +------------------+-----------------+ | root | 127.0.0.1 | | debian-sys-maint | localhost | | jack | localhost | | root | localhost | | root | russ-elite-book | +------------------+-----------------+ 5 rows in set (0.00 sec) mysql> delete from user where user = 'jack'; Query OK, 1 row affected (0.00 sec) mysql> select user,host from user; +------------------+-----------------+ | user | host | +------------------+-----------------+ | root | 127.0.0.1 | | debian-sys-maint | localhost | | root | localhost | | root | russ-elite-book | +------------------+-----------------+ 4 rows in set (0.00 sec) mysql> CREATE USER 'jack'@'localhost' IDENTIFIED BY 'test123'; ERROR 1396 (HY000): Operation CREATE USER failed for 'jack'@'localhost' mysql> CREATE USER 'jimmy'@'localhost' IDENTIFIED BY 'test123'; Query OK, 0 rows affected (0.00 sec) mysql> select user,host from user; +------------------+-----------------+ | user | host | +------------------+-----------------+ | root | 127.0.0.1 | | debian-sys-maint | localhost | | jimmy | localhost | | root | localhost | | root | russ-elite-book | +------------------+-----------------+ 5 rows in set (0.00 sec)