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While upgrading one site from Drupal 4.7.x to Drupal 5.0RC1, I encountered the following error:
Fatal error: Call to undefined function: filter_xss_bad_protocol() ... includes/common.inc on line 837
After much online research, file verification and a database restore, I found the problem: a typo.
http://example.com/update.php at the end of the upgrade, I made a typo and entered http://example.com/updates.php in the URL entry field of my browser.
Lesson learned: If you encounter the error above, check for typos. I hope this writeup helps the next person to make this typo...