If WiibaseQualityConstraints is enabled on production commons, a wbcheckconstraints call results in the following error
2020-04-01 11:50:51 [XoSAGwpAEJgAAbBdfm0AAADT] mw1394 commonswiki 1.35.0-wmf.25 exception ERROR: [XoSAGwpAEJgAAbBdfm0AAADT] /w/api.php?action=wbcheckconstraints&format=json&formatversion=2&uselang=en&id=M75908279&status=violation%7Cwarning%7Csuggestion%7Cbad-parameters Wikimedia\Rdbms\DBConnectionError from line 1418 of /srv/mediawiki/php-1.35.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php: Cannot access the database: Unknown error (10.64.48.150) {"exception_id":"XoSAGwpAEJgAAbBdfm0AAADT","exception_url":"/w/api.php?action=wbcheckconstraints&format=json&formatversion=2&uselang=en&id=M75908279&status=violation%7Cwarning%7Csuggestion%7Cbad-parameters","caught_by":"mwe_handler"} [Exception Wikimedia\Rdbms\DBConnectionError] (/srv/mediawiki/php-1.35.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php:1418) Cannot access the database: Unknown error (10.64.48.150) #0 /srv/mediawiki/php-1.35.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php(934): Wikimedia\Rdbms\LoadBalancer->reportConnectionError() #1 /srv/mediawiki/php-1.35.0-wmf.25/includes/libs/rdbms/loadbalancer/LoadBalancer.php(901): Wikimedia\Rdbms\LoadBalancer->getServerConnection(integer, string, integer) #2 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintRepository.php(35): Wikimedia\Rdbms\LoadBalancer->getConnection(integer, array, string) #3 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/CachingConstraintLookup.php(41): WikibaseQuality\ConstraintReport\ConstraintRepository->queryConstraintsForProperty(Wikibase\DataModel\Entity\PropertyId) #4 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintCheck/DelegatingConstraintChecker.php(410): WikibaseQuality\ConstraintReport\CachingConstraintLookup->queryConstraintsForProperty(Wikibase\DataModel\Entity\PropertyId) #5 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintCheck/DelegatingConstraintChecker.php(441): WikibaseQuality\ConstraintReport\ConstraintCheck\DelegatingConstraintChecker->getConstraintsToUse(Wikibase\DataModel\Entity\PropertyId, NULL) #6 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintCheck/DelegatingConstraintChecker.php(374): WikibaseQuality\ConstraintReport\ConstraintCheck\DelegatingConstraintChecker->checkConstraintsForMainSnak(Wikibase\MediaInfo\DataModel\MediaInfo, Wikibase\DataModel\Statement\Statement, NULL, array) #7 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintCheck/DelegatingConstraintChecker.php(346): WikibaseQuality\ConstraintReport\ConstraintCheck\DelegatingConstraintChecker->checkStatement(Wikibase\MediaInfo\DataModel\MediaInfo, Wikibase\DataModel\Statement\Statement, NULL, array) #8 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/ConstraintCheck/DelegatingConstraintChecker.php(154): WikibaseQuality\ConstraintReport\ConstraintCheck\DelegatingConstraintChecker->checkEveryStatement(Wikibase\MediaInfo\DataModel\MediaInfo, NULL, array) #9 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/Api/CheckingResultsSource.php(53): WikibaseQuality\ConstraintReport\ConstraintCheck\DelegatingConstraintChecker->checkAgainstConstraintsOnEntityId(Wikibase\MediaInfo\DataModel\MediaInfoId, NULL, array, array) #10 /srv/mediawiki/php-1.35.0-wmf.25/extensions/WikibaseQualityConstraints/src/Api/CheckConstraints.php(172): WikibaseQuality\ConstraintReport\Api\CheckingResultsSource->getResults(array, array, NULL, array) #11 /srv/mediawiki/php-1.35.0-wmf.25/includes/api/ApiMain.php(1591): WikibaseQuality\ConstraintReport\Api\CheckConstraints->execute() #12 /srv/mediawiki/php-1.35.0-wmf.25/includes/api/ApiMain.php(523): ApiMain->executeAction() #13 /srv/mediawiki/php-1.35.0-wmf.25/includes/api/ApiMain.php(494): ApiMain->executeActionWithErrorHandling() #14 /srv/mediawiki/php-1.35.0-wmf.25/api.php(84): ApiMain->execute() #15 /srv/mediawiki/w/api.php(3): require(string) #16 {main}