The rule was “three numbers in ascending order”. Any three. People tend to settle on “n, n+2, n+4”, try 8-10-12, then 20-22-24, collect five yeses and announce a rule that is wrong — the one try that would have settled it is the one you hoped to see refused. Wason gave the task to twenty-nine people in 1960: six named the right rule first time (Wason, 1960).
Triples accepted: 0. Refused: 0.
Wason, P. C. (1960). On the Failure to Eliminate Hypotheses in a Conceptual Task. Quarterly Journal of Experimental Psychology, 12(3), 129-140.