Database update failing for CiviCRM upgrade in Joomla 2.5
Just found the issue. In the /CRM/Upgrade/Form.php file, I found this:

static $_numberMap = array(
    0 => 'Zero',
    1 => 'One',
    2 => 'Two',
    3 => 'Three',
    4 => 'Four',
    5 => 'Five',
    6 => 'Fix',
    7 => 'Seven',
    8 => 'Eight',
    9 => 'Nine',
  );

You can see 'Fix' where it should be 'Six'. I corrected that then was able to move forward.

ALSO, I found that I had to install version 4.2.0 before I could install 4.2.19 successfully. I also upgraded to 4.1.6 prior to 4.2.0. Not sure if that was required, but I was seeing what worked.

Hopefully this will help the next poor soul that gets stuck upgrading a 6 year old CiviCRM build in Joomla!