TYPO3 Core

don't know exactly any more (5 years ago).
there was a configuration to use memcached session handling, without any memcache available. so data (user login) was written against a non working API (without error message). And so the 'written' data could not be retrieved afterwards.

The test should be: write some session data and retrieve it afterwards (on the next request). if the data could not be read something is wrong (as no login would work) => error message.