https://en.wikipedia.org/w/index.php?title=Special:ListUsers&group=monitor
Probably a group that existed at some point and got removed later but, as usual, they forgot to remove users pertaining to that group before removing it, so it now needs to be done by hand. Thankfully it's just one user, and by its naming all indicates it's from the defunct AFTv5 extension.
Thank you
Mentioned in SAL (#wikimedia-operations) [2017-09-28T19:34:10Z] <legoktm> mysql:wikiadmin@db1052 [enwiki]> delete from user_groups where ug_user =17629530 limit 1; (T176798)
mysql:wikiadmin@db1080 [enwiki]> select user_id from user where user_name="Aft5monitor"; +----------+ | user_id | +----------+ | 17629530 | +----------+ 1 row in set (0.00 sec) mysql:wikiadmin@db1080 [enwiki]> select * from user_groups where ug_user =17629530; +----------+----------+-----------+ | ug_user | ug_group | ug_expiry | +----------+----------+-----------+ | 17629530 | monitor | NULL | +----------+----------+-----------+ 1 row in set (0.00 sec) mysql:wikiadmin@db1080 [enwiki]> Bye legoktm@terbium:~$ sql --write enwiki Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 666476635 Server version: 5.5.5-10.0.28-MariaDB MariaDB Server Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved. Oracle is a registered trademark of Oracle Corporation and/or its affiliates. Other names may be trademarks of their respective owners. Type 'help;' or '\h' for help. Type '\c' to clear the current input statement. mysql:wikiadmin@db1052 [enwiki]> delete from user_groups where ug_user =17629530 limit 1; Query OK, 1 row affected, 1 warning (0.02 sec) mysql:wikiadmin@db1052 [enwiki]> select * from user_groups where ug_user =17629530; Empty set (0.00 sec)