We try to notify this list when API changes are made in Drupal 7, so contrib and custom developers have a chance to fix their code before they have the pain of figuring out why it's broken.

File, Form, and XMLRPC APIs changed recently:

• (issue) If you use #limit_validation_errors in a multistep form, especially #limit_validation_errors = array(), your multistep form may now be broken. 
• (issue) If you do file downloads, there is a new access check regimen to use. hook_file_download_access() and hook_file_download_access_alter() have been added.
• (issue) hook_file_references() was removed, and the entire method of managing references to managed files was changed and simplified. The summary is in http://drupal.org/node/353458#comment-3193206. If your code implements hook_file_references() or otherwise uses managed files, you'll want to take a look at this.
• (issue) xmlrpc() function was changed.  If your module uses xmlrpc(), it's broken until you fix it. Upgrade details are at http://drupal.org/update/modules/6/7#xmlrpc

Thanks,

-Randy

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Randy Fay
Drupal Module and Site Development
randy@randyfay.com
+1  970.462.7450