Here’s a very interesting set of comments from Linus Torvalds and Theodore Tso on the problem with writing specs:

kerneltrap.org/node/5725

Here’s a great quote from Linus:

 The classic example of this is the OSI network model protocols.
 Classic spec-design, which had absolutely _zero_ relevance for the
 real world.We still talk about the seven layers model, because it's
 a convenient model for _discussion_, but that has absolutely zero to
 do with any real-life software engineering. In other words, it's a
 way to _talk_ about things, not to implement them.

 And that's important. Specs are a basis for _talking_about_ things.
 But they are _not_ a basis for implementing software.

And a good one from Ted Tso:

 In those cases, if you implement something which is religiously
 adherent to the specification, and it doesn't interoperate with the
 real world (i.e., everybody else, or some large part of the
 industry) --- do you claim that you are right because you are
 following the specification, and everyone else in the world is
 wrong? Or do you adapt to reality?

And another one from Linus:

 So don't talk about specs. Talk about working code that is
 _readable_ and _works_. There's an absolutely mindbogglingly huge
 difference between the two.

All heresy to the BDUF school of thought, of course. But relatively uncontroversial in the XP world. Interesting to see the parallel evolution.