I hit the same issue - this is a bug for sure.

Here are my findings:
 - PHP Version - 5.3 - no bug, 5.3+ the bug is there
 - OS - Linux Gentoo (not tested on other)
 - Observation/note: The issue seems to be related to that a property with same name ("_isSourceDirty" in my case and "errors" in previous example) is defined in both of the classes, no matter what the value is, if not the assertions does not fail. My example is extraction from Zend_CodeGenerator package (Zend Framework 1) however such usage is a very common case in typical inheritance usage
 - Workaround: Avoid usage of "loose" code, unless you want to shoot your self in the leg J

Test script:
---------------------------------------------
<?php
class Zend_CodeGenerator_Abstract
{
    protected $_isSourceDirty = true;
}

class Zend_CodeGenerator_Php_Abstract extends Zend_CodeGenerator_Abstract
{
    protected $_isSourceDirty = true;
}

class Zend_CodeGenerator_Php_Docblock extends Zend_CodeGenerator_Php_Abstract
{
    protected $_shortDescription = null;
    
    public function setShortDescription($shortDescription)
    {
        $this->_shortDescription = $shortDescription;
        return $this;
    }
}

$d1 = new Zend_CodeGenerator_Php_Docblock();
$d1->setShortDescription('Short Desc One');

$d2 = new Zend_CodeGenerator_Php_Docblock();
$d2->setShortDescription('Short Desc Two');

// according to http://www.php.net/manual/en/language.oop5.object-comparison.php assertions should pass all

// strict comparison, expecting false
assert(false === ($d1 === $d2));

// loose comparison, expecting false
assert(false === ($d1 == $d2));

var_dump(array(
    'd1' => $d1,
    'd2' => $d2
));

This changed since 5.4, as can be seen from this 3v4l:

http://3v4l.org/TJWQR