Anyone would consider it a stroke of bad luck to be pulled over for driving six miles per hour over the speed limit, but Roy Caballes had an additional reason to curse his ill fortune. On the November afternoon in 1998 when an Illinois state trooper stopped him, Caballes was carrying 282 pounds of marijuana in his trunk. At first it looked like he’d get off with just a warning. Then another officer pulled up and swept his car with one of the most advanced pieces of technology then available to law enforcement: a drug-sniffing dog named Krott. The pooch...