I could go round randomly reassuring people they will not lose their sight. If I reassure enough people, through random chance, I'm likely to find myself reassuring people in danger of losing their sight. A proportion of those actually will not lose their sight. A proportion will. The former will think I am some kind of prophet. The latter will think I'm full of crap. No miracles needed, as long as they don't know about each other.
And you could improve your odds by being observant about people, and targeting those who might have strong glasses, or be acting as if they had recent sights issues.