This question is motivated by a question which came up recently on engineering.SE: https://engineering.stackexchange.com/questions/3186/cracks-on-the-wall. It may be closed in the future so I will summarize. The user posted pictures of (very large) cracks in the wall of his house and asked the community "could you please advice to do continue the construction as per proper standard or I have demolish the house and re-built again?"
A professional engineer advising this individual under the auspices of a hired firm would have certain legal obligations to the correctness of their opinion. If one were to advise this individual in a way that led to him living in an unsafe structure on Stack Exchange, what legal repercussions could they face?
In particular, is there any legal precedent (read prior court case) which absolves freely offered online advice from liability?