In ExpressionEngine version 1.6, a new module was added: Pages.  Prior to this release, a third party module was available by the same name (Mark Huot’s Pages module).  Due to naming collisions and file dependencies in the code, if you are using this third-party module you must update it before upgrading to ExpressionEngine 1.6 or you risk losing your existing module data and/or breaking your site.

Instructions for updating Mark Huot’s Pages module to the newer version (now named “Tome") can be found here: http://docs.markhuot.com/ee/modules/tome/

Note to Developers: This scenario demonstrates the importance of giving unique class and table names to your add-ons, and avoiding unchecked file dependencies.  The suggested naming convention is to use identifying prefixes for all class, file, and table names.  In so doing, you will avoid potential collisions with ExpressionEngine native classes, conflicts with other third party developers’ add-ons, and even potential conflicts with non-ExpressionEngine based PHP scripts that users may be running on their sites.