Possible timing issue with test phpunit/api/v3/AttachmentTest::testCreate
What I think is happening is that when it first calls
$this->callAPISuccess('Attachment', 'create'... the fcs hash is generated using the current value of time(). Then lower down when it calls
$this->callAPISuccess('Attachment', 'get',... the fcs hash gets generated again via _civicrm_api3_attachment_format_result() which calls CRM_Core_BAO_File::generateFileHash() again, and so the returned value will depend on the value of time() at that point.
While almost all the time those two values of time() will be the same because it's very fast, if it happens to cross over the second boundary the second value will be different than the first and the test fails.