There are two book bags each containing 10 poker chips. In one bag there are 7 red and 3 blue. In the other bag there are 3 red and 7 blue. Five chips are drawn out of one of the bags and shown to the subject (one at a time then returned to the bag). The subject does not know which bag the chips came from. There is an equal chance that the draws are made from either bag. After each draw the subject reports which bag he believes the chips are coming from and provides a probability that the chips are being drawn from that bag.
The problem comes from the early "revision of judgment" work that indicated that people were conservative with respect to Bayes.