It's called professional responsibility, and it's taken seriously by lawyers and insurance companies looking for a scapegoat. I've gone so far as to sit in the back of a boat, as far from the helm as I could get, with my arms folded when an owner refused to heed my warnings. Just recently watching the People's Court (I have some free time this time of year), I saw 3 cases where contractors claimed to have told homeowners that they shouldn't do what they wanted to do. The homeowner insisted so the contractor complied. In each case it turned out badly and the contractor was held liable.