When you upgrade from Drupal 5 to Drupal 6 you might run into the following error if the upgrade was not properly done
Warning: Unknown column 'serialized' in 'field list' query: SELECT data, created, headers, expire, serialized FROM cache WHERE cid = 'variables' in [drupal 6 "/includes/database.mysqli.inc" file] on line 128
There could be several reasons why the error happened but there is a simple solution to solving this problem.