When upgrading to Magento v1.4+ from a previous version you may get an error "Invalid mode for clean() method".
This occurs because of a change in the Zend Framework shipped with Magento.
To fix the error, delete the /app/code/core/Zend/Cache directory.
Note: With Magento v1.4.0.1, error reporting is switched off. To switch error reporting on, rename /errors/local.xml.sample to /errors/local.xml. Rename it back again on production websites.