1.43. The king dies and two men both claim to be his long-lost son. One of the king's advisors proposes a test to determine the identity of the true heir. One claimant agrees to the test; the other refuses. The one who agreed is sent packing; the one who refused is identified as the rightful heir. (SP, from The Next Book of OMNI Games; earlier from How Come?)
1.43 answer: The proposed test was a blood test. Knowing that blood typing isn't very accurate, the impostor felt it was worth trying to get away with it. But the true heir was a hemophiliac and couldn't give blood for the test.