Question for other freelancers: how do you handle changes in a spec?

I've been nervous about this lately because I'm doing freelance work and the contract outlined a spec that we've now deviated from greatly. I'm not incredibly worried because we do bi-weekly billing but do you regularly ask clients to revise contracts if the spec changes?

And when you begin a contract, what level of specificity in a spec is a good idea? Obviously some level of specificity is a must, but an overly detailed contract leaves you in the position I described above where the contract does not actually describe the work in progress.