Seen e.g. in https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-php72-docker/24311/console:
PHP Fatal error: Declaration of Wikibase\Client\Tests\DataAccess\Scribunto\Scribunto_LuaWikibaseLibraryTestCase::tearDown() must be compatible with Scribunto_LuaEngineTestBase::tearDown(): void in /workspace/src/extensions/Wikibase/client/tests/phpunit/includes/DataAccess/Scribunto/Scribunto_LuaWikibaseLibraryTestCase.php on line 224
That's because we're upgrading to PHPCS 28.0.0, which forces you to add :void typehints to setUp and tearDown (because they were added in PHPUnit 8). Scribunto was already upgraded, hence it now has the typehints. But Wikibase extends some of those Scribunto classes without using typehints, and hence it broke.
A quick way to unbreak the world would be to upgrade wikibase-codesniffer to use mediawiki-codesniffer 28.0.0, then run phpcbf.