There is a new hotifix update released for AutoCAD 2006 based products.
Link to Update

This update corrects two problems that have been reported by customers with regards to editing CUI files.

* Duplicate IDs in a CUI file cause an error message indicating an invalid file.

* Menu files that contain a screen menu are incorrectly converted, resulting in extra spaces at the top.

Upon converting a menu file that contains a screen menu, erroneous blank lines are added at the top of the screen menu section. This can affect the display of commands that appear toward the bottom of the screen menu.

Description of Problem
Editing a CUI file could introduce duplicate entries (IDs) that may cause the file to become corrupt. Loading the corrupt CUI file results in an error message stating that the file is invalid and you can no longer customize the file.

Affected Products
* AutoCAD® 2006
* Autodesk® Architectural Desktop 2006
* Autodesk® Building Systems 2006
* Autodesk® AutoCAD® Revit® Series 8.0 - (for AutoCAD 2006 when installed as part of Autodesk AutoCAD Revit Series 8.0.)
* Autodesk Map® 3D 2006
* Autodesk® Civil 3D® 2006
* Autodesk® Land Desktop 2006
* AutoCAD® Electrical 2006
* AutoCAD® Mechanical 2006
* Autodesk Mechanical Desktop® 2006
* Autodesk Inventor® 10 - (for AutoCAD Mechanical 2006 or Autodesk Mechanical Desktop 2006 when installed as part of Inventor 10.)