I'm installing a custom component. The install package had some errors in it, which should throw warnings in Installer. However, I discovered that when 'Debug Site' is on in Global Site Configuration, the errors are suppressed by the following code in libraries/joomla/installer/installer.php starting at line 385:

    $conf = JFactory::getConfig();
    $debug = $conf->get('debug');

    if ($debug)
    {
        throw new RuntimeException('Installation unexpectedly terminated: ' . $msg, 500);
    }

In my case, the $msg variable is empty, so this throws a rather meaningless error. If 'Debug Site' is off, the site shows some meaningful warnings.

For information, here is the version I'm running:

Joomla! Version  Joomla! 3.0.2 Stable [ Ember ] 08-November-2012 14:00 GMT
Joomla! Platform Version     Joomla Platform 12.2.0 Stable [ Neil Armstrong ] 21-September-2012 00:00 GMT