Radley Balko writing for Tech Central Station on We're From the Government and We're Here to Help:
Dan Peruchi, father of four, enjoyed fixing up old cars and reselling them. Because the dealers he worked with dealt mainly in cash, he usually had lots on hand. Peruchi was driving home to Ft. Worth, Texas when he noticed the flashing lights of a police car behind him. After pulling him over, the officer asked to search Peruchi's car. Peruchi had about $19,000 in a satchel, but nothing criminal to hide, so he consented. The officer found Peruchi's cash, and immediately suspected Peruchi was involved with drugs. He called in drug-sniffing dogs, who then reacted suspiciously to Peruchi's satchel (most all of the U.S. money supply carries feint amounts of drug residue, mostly cocaine).
The dogs' reaction, no more, was enough for the West Memphis police department to seize Peruchi's money. When Peruchi protested, the police officer retorted, "Carry checks next time."
Read the rest of Radley's excellent commentary here.