I disagree in the general. It's very easy to write a spec that contradicts itself and is impossible to actually implement. Some things do have to be designed,

I disagree in the general. It's very easy to write a spec that contradicts itself and is impossible to actually implement. Some things do have to be designed, and not just thrown together though. That may be through spec, but it could also be working things through on paper. Concurrency is certainly one of these. Other things need to be iterated on in code. I'd hate to play a platformer where the movement mechanics were specced out by a comitee before any code was made.