Deborah Melkin tells a story of a design battle she lost:
The programmers came to me and said we need to add a large number of columns to this table for one piece of functionality. It would more than double the total number of columns on the table. Oh, and all of the new columns would be NULL since we would only need to populate them if they were using that functionality and even then, not all of them would require data. The final result would be that 65-75% of the table would end up having nullable fields with the majority of those having NULL for the value.
I said what I think any sane DBA would say to this request: No.
Click through for the rest of the tale.