How do you respond to "I told you so"?

Someone warns you that what you’re doing will have a bad consequence, but you ignore them, thinking you know better. Later on, it turns out that their warning was correct, how do you behave towards that person?

A lone voice warns of impending disaster to the planet if we keep on doing this or that, and we call the person crazy and put the person in the insane asylum. Later on the disaster happens exactly as predicted. What would we do with that person if they were still alive? What if they were now dead?

Can you provide any examples of these kinds of things from your own life?