In the worker run internals the concrete worker is run (in ZfExtended_Worker_Abstract::_run the $this->work call), afterwards the worker is updated (save call) to state "done".
Since "work" can need a long time, the mysql connection can go away. It should be ensured after ->work that mysql connection is still alive or should be reopened!
This happened in real live with the stat worker for a huge task, the stat worker died with a mysql gone away message.