If you delete user and remove his posts, post_edit_user is not updated to ANONYMOUS for posts which were edited by deleted user. This causes errors in viewtopic, like

[phpBB Debug] PHP Notice: in file [ROOT]/viewtopic.php on line 1434: Undefined index: 57

[phpBB Debug] PHP Notice: in file [ROOT]/viewtopic.php on line 1434: Undefined index: 57

where 57 is deleted user id (just for example).