If this means, "You're mistaken," then I say maybe I am wrong. My cousin, who did not smoke, although her husband did, died from lung cancer in her 30's. The family's assumption was that her husband's smoking was the cause. I admit this falls far short of being proven, and I'll be more careful in the future about making categorical statements like this.
I don't believe you.
If this means, "You're making this up," then I say, "What an odd thing to say!"