In this presentation at Mix08 on the development of the Microsoft Office 2007 'ribbon' interface, Jensen Harris, design lead for the MS Office User Experience team provides an excellent example of how a UI paradigm can reach a point, where it has to be overhauled completely, because it can no longer sustain the increasing functionality of the product. In the 45 minute presentation he very openly discusses Microsoft's struggle of fitting all (new) features into the existing user interface, leading to 'interesting' UI elements such as hidden menu items, task panes and paperclips.

See also:
> Adaptive path interviews Jensen Harris about the ribbon
> No new features in Office 2007

[Via HBR blogs]