I recently came across the following error after using the SQL Server 2005 Index Tuning Wizard:

A column has been specified more than once in the order by list.  Columns in the order by list must be unique.

I assumed the error was caused by a line in one of my stored procedures or triggers and I spun my wheels for quite some time looking for the root cause.  As it turns out,  the tuning wizard added several hypothetical indexes to my table that weren't displaying when I tried to view the indexes the traditional way (right clicking on the table and choosing 'manage indexes'.  To see them, I had to run the following stored procedure:  sp_helpindex ('table') 

After dropping all of the hypothetical indexes,  the issue was resolved.