If you’re using the Shared Users Table trick to create an ad-hoc blog network, then you need to be using WP-Orphange. When you set the constants CUSTOM_USER_TABLE and CUSTOM_USER_META_TABLE in your wp-config.php file, your users can log into any blog that defines the same Users table. However, if the blog isn’t the one they registered on they won’t be assigned any Roles or Capabilities on the other blogs, and they will be shown an error. I call these users ‘orphans.

WP-Orphanage is a WordPress plugin that automatically adopts all of your orphans automatically, and will set a Role for them on the blog they are trying to log into. You have control over which Role they are set to in wp-admin.

The v1.1 update fixes a bug introduced in the option screen in v1.0.1 and is recommended for all users of this plugin.

Many thanks to Vince from Alphak.net – Born to Be Wired for reporting this error and helping me debug the solution.