Spotted while testing https://gerrit.wikimedia.org/r/#/c/431485/
Stacktrace can be found at https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-hhvm-docker/1227/console and error details copied below as follows:
23:16:55 1) MergeUserTest::testDeleteUser 23:16:55 Wikimedia\Rdbms\DBQueryError: A database query error has occurred. Did you forget to run your application's database schema updater after upgrading? 23:16:55 Query: UPDATE `unittest_site_stats` SET ss_users=ss_users-1 23:16:55 Function: SiteStatsUpdate::tryDBUpdateInternal 23:16:55 Error: 1690 BIGINT UNSIGNED value is out of range in '(`wikidb`.`unittest_site_stats`.`ss_users` - 1)' (/tmp/quibble-mysql-5gyps4x0/socket) 23:16:55 23:16:55 23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1449 23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1419 23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:1192 23:16:55 /workspace/src/includes/libs/rdbms/database/Database.php:2060 23:16:55 /workspace/src/includes/deferred/SiteStatsUpdate.php:132 23:16:55 /workspace/src/includes/deferred/MWCallableUpdate.php:34 23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:268 23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:226 23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:134 23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:307 23:16:55 /workspace/src/includes/deferred/DeferredUpdates.php:100 23:16:55 /workspace/src/extensions/UserMerge/MergeUser.php:514 23:16:55 /workspace/src/extensions/UserMerge/MergeUser.php:61 23:16:55 /workspace/src/extensions/UserMerge/tests/phpunit/MergeUserTest.php:73 23:16:55 /workspace/src/tests/phpunit/MediaWikiTestCase.php:423 23:16:55 /workspace/src/maintenance/doMaintenance.php:94