Years ago someone asked How can I enforce uniqueness based on a condition in SQL Server? to which I suggested a Filtered Index.
Unfortunately, my answer is irrelevant to the OP because it doesn't apply to sql-server-2005 (my mistake). That said, the answer is pretty popular and it continues to earn upvotes -well, until it draws meta's scouring gaze.
So, do I have a responsibility to remove that answer? How can the community best be served here?