This should be elementary to most people: if you sell your hard drive, you should make sure to completely erase its contents. At least one police department didn't know that since a student in Potsdam, Germany, bought their hard drive on ebay and it contained loads of very sensitive police information.
Opening eyes to hackers:
"Europeans bracing for rise in data theft
BERLIN Last month, a student in Potsdam, southwest of Berlin, got more than he bargained for after buying a used computer hard drive for [Euros] 20 on the eBay auction Web site.
The hard drive, worth about $26, contained confidential data from the local police, including procedures for hostage-takings, SWAT-team staffing and an analysis of current threats..."