Fixed it by querying the wikis I was attached on, comparing it to all.dblist-labs and killing the localuser entries for those which don't exist:

sudo -u apache mwscript eval.php --wiki=enwiki

CentralAuthUser::getCentralDB()->delete( 'localuser', array( 'lu_name' => 'Krenair', 'lu_wiki' => array( 'cswiki', 'cswikibooks', 'cswikinews', 'cswikisource', 'dewikinews', 'frwiki', 'incubatorwiki' ) ) );

Other people probably have similar issues though.

sigh, still get it on logging in for some reason...

$db = CentralAuthUser::getCentralDB();
$wikiList = CentralAuthUser::getWikiList();
$db->delete( 'localuser', array( 'lu_name' => 'Krenair', 'lu_wiki NOT IN (' . $db->makeList( $wikiList ) . ')' ) );
$db->delete( 'localnames', array( 'ln_name' => 'Krenair', 'ln_wiki NOT IN (' . $db->makeList( $wikiList ) . ')' ) );

I can log in now

• Bug 66401 has been marked as a duplicate of this bug. ***

Reusing topic of duplicate bug 66401

There's some slightly weird cases around the code I posted. Some quotes from IRC so they aren't missed.

<Krenair> on deployment-prep, if I run this query:
<Krenair> show databases where Database not in ('arwiki','cawiki','commonswiki','deploymentwiki','dewiki','ee_prototypewiki','en_rtlwiki','enwiki','enwikibooks','enwikinews','enwikiquote','enwikisource','enwikiversity','enwiktionary','eowiki','eswiki','fawiki','hewiki','hiwiki','jawiki','kowiki','loginwiki','metawiki','ruwiki','simplewiki','sqwiki','testwiki','ukwiki','wikidatawiki','zhwiki');
<Krenair> (that list of wiki DBs is from CentralAuthUser::getWikiList())
<Krenair> I get some entries back which should be listed by CentralAuthUser::getWikiList()....
<Krenair> At least aawiki, not sure about others
<Krenair> these look like wiki dbs: auth, dewikinews, dewikivoyage, enwikivoyage, labswiki
<Krenair> no idea what/where auth or labswiki are supposed to be. dewikinews/dewikivoyage/enwikivoyage return "No wiki found" at the normal hostnames.

(Additionally, centralauth.localnames/localusers seems to have a ton of references to wikis that AFAIK have never been in deployment-prep?)

• Bug 72773 has been marked as a duplicate of this bug. ***

From a mail Bryan Davis sent me a while ago:

When folks reported not being able to log in to deployment.beta and
Chad and I both realized "wiki rename" at the same time here's what I
did to try and fix:

Using this script:

$ cat betasql
#!/bin/sh

USER=`php -r 'include("/a/common/wmf-config/PrivateSettings.php");
print $wgDBuser;'`
PASS=`php -r 'include("/a/common/wmf-config/PrivateSettings.php");
print $wgDBpassword;'`
HOST="deployment-db1"
mysql -u $USER -p$PASS -h $HOST

Then in the mysql prompt:

> use centralauth
> update localuser set lu_wiki = 'deploymentwiki' where lu_wiki = 'labswiki';
Query OK, 940 rows affected (0.39 sec)
Rows matched: 940  Changed: 940  Warnings: 0

> update localnames set ln_wiki = 'deploymentwiki' where ln_wiki = 'labswiki';
Query OK, 943 rows affected (0.54 sec)
Rows matched: 943  Changed: 943  Warnings: 0

> update globaluser set gu_home_db = 'deploymentwiki' where gu_home_db = 'labswiki';
Query OK, 84 rows affected (0.49 sec)
Rows matched: 84  Changed: 84  Warnings: 0

I also looked at wikiset.ws_wikis and renameuser_status.ru_wiki but
neither of them mentioned the old labswiki name.

select distinct(lu_wiki) from localuser;

463 rows in set (0.01 sec)

var_dump(count(CentralAuthUser::getWikiList()));

int(30)