Painless Functional Specifications - Part 1: Why Bother?
http://www.joelonsoftware.com/articles/fog0000000036.html

Spolsky, Joel
Joel on Software
2000

Abstract:

Why won't people write specs? People claim that it's because they're saving time by skipping the spec-writing phase. They act as if spec-writing was a luxury reserved for NASA space shuttle engineers, or people who work for giant, established insurance companies. Balderdash.