You may get this error while rebuilding index on Magento.
Fatal error: Uncaught exception 'Mage_Core_Exception' with message
'Catalog URL Rewrites Index process is working now. Please try run this
If index request stop/timeouts/fails
because of a PHP error, the indexer lock files lying around even though
there's no indexing going on. These files are located in
How to fix:
just delete the files and restart your indexing