With my main account, I opted in, out, in, out at beta cluster (over about 10 minutes, with pauses between each).
The final opt-out, resulted in this error:
[7375dc8e] /wiki/Special:Preferences MWException from line 3711 of /srv/mediawiki/php-master/includes/User.php: CAS update failed on user_touched for user ID '1255' (read from slave); the version of the user to be saved is older than the current version. Backtrace: #0 /srv/mediawiki/php-master/includes/Preferences.php(1439): User->saveSettings() #1 /srv/mediawiki/php-master/includes/Preferences.php(1450): Preferences::tryFormSubmit(array, PreferencesForm) #2 /srv/mediawiki/php-master/includes/htmlform/HTMLForm.php(591): Preferences::tryUISubmit(array, PreferencesForm) #3 /srv/mediawiki/php-master/includes/htmlform/HTMLForm.php(503): HTMLForm->trySubmit() #4 /srv/mediawiki/php-master/includes/htmlform/HTMLForm.php(518): HTMLForm->tryAuthorizedSubmit() #5 /srv/mediawiki/php-master/includes/specials/SpecialPreferences.php(70): HTMLForm->show() #6 /srv/mediawiki/php-master/includes/specialpage/SpecialPage.php(384): SpecialPreferences->execute(NULL) #7 /srv/mediawiki/php-master/includes/specialpage/SpecialPageFactory.php(553): SpecialPage->run(NULL) #8 /srv/mediawiki/php-master/includes/MediaWiki.php(249): SpecialPageFactory::executePath(Title, RequestContext) #9 /srv/mediawiki/php-master/includes/MediaWiki.php(682): MediaWiki->performRequest() #10 /srv/mediawiki/php-master/includes/MediaWiki.php(476): MediaWiki->main() #11 /srv/mediawiki/php-master/index.php(41): MediaWiki->run() #12 /srv/mediawiki/w/index.php(3): include(string) #13 {main}
Since that, I get the same error every time I try to save my preferences, no matter which setting I try to change.
See also, the possibly related: T95839: CAS update failed on user_touched for user ID