Getting the specs right is a fallacy (Lehman-Belady's Law)

"A classic study by Lehman and Belady identified several 'laws' of system change.

Continuing change > A program that is used in a real-world environment must change, or become progressively less useful in that environment.

Increasing complexity > As a program evolves, it becomes more complex, and extra resources are needed to preserve and simplify its structure

Studies have shown that more than half of total software budgets are devoted to 'software maintenance'. Furthermore, there are many claims that the cost of fixing errors in software projects goes up over time. This suggests that one should try to catch errors as early as possible, and perhaps one should invest more effort into 'getting requirements right'. The latter conclusion is a fallacy, however, if we recall that Lehman's laws tell us that requirements will always change."

-- From Oscar Nierstrasz, at a delicious Smalltalk course @  scg.unibe.ch/teaching/sma

-- See the Wikipedia article on Lehman's Law