MetaWiki autocreated, detached accounts
See bug 29234, bug 35792, bug 39060, bug 39996, bug 39997 and friends for context.
One example silly query one can run e.g. on toolserver (sql metawiki_p):
log_title, log_user, log_timestamp
JOIN user ON logging.log_action = 'autocreate' /*
AND log_timestamp > 20130431000000 */
AND logging.log_user = user.user_id
WHERE user.user_name NOT IN (
SELECT lu_name FROM centralauth_p.localuser lu WHERE lu.lu_wiki = 'metawiki'
15k results on Meta-Wiki only.
An actual query needs to be made slightly faster, I suppose, and actually check for the validity of the local account (it can be attached but nevertheless unusable, with password and email not working) and of its global username (which might have been deleted, or detached on purpose).
Note, at some point James said this would be done as part of the work for bug 35707.