Scenario: you've developed a magento extension, tested it on your development server for all these potential fringe cases, even brought in live customer data+code to test against... finally you deploy it to the live site, only to find that another (3rd party) extension uses the same dirty <rewrite>s as your code does. Damn.
What a great occasion to learn all about resolving namespace conflicts in magento. Here's my list of links:
This is really all you need to know: http://www.webshopapps.com/blog/2010/11/resolving-magento-extension-conflicts/
If you don't know how to use your text editor, you might need to use this extension for help: http://www.maisondulogiciel.com/english/magento/magento-extension-conflict.html
And finally, here's an excellent / slightly confounding exposé: http://magebase.com/magento-tutorials/magento-extension-clashes-winners-and-loosers/