In a case when WikibaseRepo (wikidata) has Lexeme extension enabled, and WikibaseClient (wikipedia) does not, and they share the object cache where EntityRevision objects are stored, on the read on the Client side fatal errors happen due to unserializing unknown class (LexemeId in this case)
See log bit from test.wikipedia:
Catchable fatal error: Argument 1 passed to Wikibase\Client\Usage\UsageTrackingSnakFormatter::addLabelUsage() must be an instance of Wikibase\DataModel\Entity\EntityId, __PHP_Incomplete_Class given in...
Repo and Client share cache prefixes, thus whatever Repo puts in the cache, is read by Client. In case both systems run different version of software (e.g. one has WikibaseLexeme enabled, and the other not), Client code is bound to deal with unknown code.
Related to: T195615.