This occurs when registering a new account on english beta wikipedia.
[W-WBqKwQBGoAADimdgUAAAAQ] /w/index.php?title=Special:CreateAccount&returnto=Main+Page UnderflowException from line 446 of /srv/mediawiki/php-master/extensions/ConfirmEdit/FancyCaptcha/includes/FancyCaptcha.php: Ran out of captcha images Backtrace: #0 /srv/mediawiki/php-master/extensions/ConfirmEdit/SimpleCaptcha/SimpleCaptcha.php(1126): FancyCaptcha->getCaptcha() #1 /srv/mediawiki/php-master/extensions/ConfirmEdit/includes/auth/CaptchaPreAuthenticationProvider.php(60): SimpleCaptcha->createAuthenticationRequest() #2 /srv/mediawiki/php-master/includes/auth/AuthManager.php(2095): CaptchaPreAuthenticationProvider->getAuthenticationRequests(string, array) #3 /srv/mediawiki/php-master/includes/auth/AuthManager.php(2073): MediaWiki\Auth\AuthManager->getAuthenticationRequestsInternal(string, array, array, User) #4 /srv/mediawiki/php-master/includes/specialpage/AuthManagerSpecialPage.php(256): MediaWiki\Auth\AuthManager->getAuthenticationRequests(string, User) #5 /srv/mediawiki/php-master/includes/specialpage/LoginSignupSpecialPage.php(141): AuthManagerSpecialPage->loadAuth(NULL) #6 /srv/mediawiki/php-master/includes/specialpage/LoginSignupSpecialPage.php(229): LoginSignupSpecialPage->load(NULL) #7 /srv/mediawiki/php-master/includes/specialpage/SpecialPage.php(569): LoginSignupSpecialPage->execute(NULL) #8 /srv/mediawiki/php-master/includes/specialpage/SpecialPageFactory.php(558): SpecialPage->run(NULL) #9 /srv/mediawiki/php-master/includes/MediaWiki.php(288): MediaWiki\Special\SpecialPageFactory->executePath(Title, RequestContext) #10 /srv/mediawiki/php-master/includes/MediaWiki.php(862): MediaWiki->performRequest() #11 /srv/mediawiki/php-master/includes/MediaWiki.php(517): MediaWiki->main() #12 /srv/mediawiki/php-master/index.php(42): MediaWiki->run() #13 /srv/mediawiki/w/index.php(3): include(string) #14 {main}