The question Example of function which is not computable was recently edited by the original poster to completely change the meaning. In particular, the accepted answer is no longer reflective of the question and is misleading as an answer to the current question. I'm not sure what our practice is for this sort of situation.
Depending on the situation, I think we should either:
1) Roll back the change, and ask OP to add a new question, linking to the old.
2) Make the edits which ensure that the original question still remains. In this current case, that is easily done. In the general case, when it might not be possible to just make additions to the question, we request OP to create a new question and rollback any changes which change the meaning of the question substantially, pointing OP to this thread.
We should also encourage people who answer to not immediately change their answer and if they do so, they only add instead of taking away.