Did someone here ask about the name of a fraud where the fraudster makes a number of true predictions for free, then says "no more predictions, I'm selling my system."? There's no system, instead the fraudster divided the potential victims into groups, and each group got different predictions. Eventually, a few people have the impression of an unbroken accurate series.

Anyway, the scam is called The Inverted Pyramid, and the place I'd seen it described was in the thoroughly charming "Adam Had Three Brothers. by R.A. Lafferty.

Edited to add: It turned out that someone had asked at Making Light.