Surprisingly, after testing succesfully locally unit tests, I've found this error in Travis, in unit tests performed by \Magento\Customer\Test\Unit\Model\AccountManagementTest class:

Simply rebuilding the job worked and all the tests passed, but this error may happen again at any time, due to how are mocks built in that class:

• A datetime variable is set from current time:

• Below, customerSecure mock expects setRpTokenCreatedAt method to be called with previous datetime value will return self:

• But testing the method, the customerSecure::setRpTokenCreatedAt method is called with the current datetime, that may be NOT the previous one, so tests fails because method was not expected to be called with new date value:

• The test assumes the mock creation and the mocked method call will happen in the same second, thing that may not be true and it's irrelevant for tests using \Magento\Customer\Test\Unit\Model\AccountManagementTest::prepareInitiatePasswordReset method:

Removing expected date fix this rare, weird, but actually happening, issue.

Manual testing scenarios

1. To simulate this, change
   $dateTime = date(\Magento\Framework\Stdlib\DateTime::DATETIME_PHP_FORMAT);
   to
   $dateTime = '2017-10-20 00:00:00';
   in \Magento\Customer\Test\Unit\Model\AccountManagementTest::prepareInitiatePasswordReset method.
2. Run unit test over this class.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)