Surveillance is two-sided, and its benefits must be acknowledged. Yet at the same time risks and dangers are always present in large-scale systems and of course power does corrupt or at least skews the vision of those who wield it. Large-scale technological infrastructures are peculiarly prone to large-scale problems. And especially where computer systems are concerned, one inadvertent or ill-advised keystroke can easily cause havoc. Think of the release for 'research' purposes, of twenty million of ordinary peoples' online search queries from AOL in August 2006. Supposedly shorn of identifiers, it took only moments to start connecting search records with names. This report looks at some problems of large-scale surveillance systems.