Upgrading from Drupal 4.6 has always been complicated by issues of character sets, but the Drupal upgrade scripts normally solve most of these problems for us. But when your dealing with a complex upgrade you need a good understanding of how Drupal is dealing with character sets in Mysql.
A rather obscure one this, but we found a problem today on one of our Drupal sites with the filter form on the node admin screen (admin/content/node). Changing the filter settings on the form had no effect, which prevented you from filtering the list of nodes. We tracked the solution down to the $_SESSION array being cleared between page reloads, and a quick google pointed the finger at the Remember me module. Disabling the module solved the problem
We make heavy use of the Drupal image assist module (or img_assist as its known) - but find that its handling of empty alt tags is quite poor. The problem being that if when inserting the image the user does not specify a caption or description then the alt (and title) attributes are both empty.
Building multilingual sites in Drupal using the i18n module always throws up a few fun problems to be solved. One of the most awkward being views.
The i18n views integration module does an excellent job of allowing you to return only nodes matching a particular language, but it doesn't help when it comes to translating the view itself - and more importantly allowing a view to site nicely into multiple positions in the menu (one per language).
Well, it's finally happened - the brand spanking new Drupal 6 has been released into the wild. We have been playing a lot with Drupal 6 and getting ourselves quite excited (finally drupal_execute has decent return values, and the menu system feels a bit less 'brutal'). However, while we would love to start building client sites on it there is one major omission - the views module.
Just flagging this as a bit of an issue - it appears there is fairly serious bug in the version of the filter module provided in Drupal 5.6 which prevents you from saving changes to the allowed HTML tags
A patch is available on drupal.org but it's quite well hidden! Post number 7 in this thread contains a working patch http://drupal.org/node/208700