Debugging cron issues and errors is one of the most tricky things in Drupal. If you are dealing with a big site, with people adding nodes, inserting content which you can't allways force to be commited in the right way, and things like that, it is not unusual to find a cron broken by some "misterious" node.
Well, it is quite easy to find this "misterious" offender, just logging the cron activity. How, very easy, with my new module "cron_killing_finder".
It is published in my "Drupal SandBox": http://drupal.org/sandbox/programadoresweb/1895746