Edward Snowden’s recent leaks of the NSA’s telephony metadata collection program, and the Internet surveillance programs PRISM and XKeyscore are only the latest iterations of the “big data” phenomenon. Arriving just in time for 9/11, new technologies have enabled government agencies to collect and aggregate massive amounts of information, usable in counterterrorism and domestic law enforcement alike. While such moves have probably stopped some terrorist plots, they also entail systemic inefficiencies that lead unavoidably to unjust results, in the form of both false positives and false negatives. This Article explains these inefficiencies by describing a complex positive feedback loop inherent in domestic counterterrorism law enforcement.