One of the common arguments against mass surveillance is that it could backfire: you might collect so much data that finding crucial info becomes difficult. As it turns out, that's a well-founded theory. A 2010 UK report leaked by Edward Snowden warned that MI5 spies were collecting so much data that there was a real risk of an "intelligence failure" where it would miss info that could save lives. Without enough staff and tools, it simply couldn't handle the sheer glut of raw surveillance content.
GCHQ's snoops have had problems, too. In 2009, a study revealed that 97 percent of data collected on surveillance targets hadn't even been seen. There was a real risk that something important might slip between the cracks, according to the findings. And there are more privacy concerns than previously thought. GCHQ would later offer internet metadata to other British agencies (such as MI5 and tax agents) as part of a sharing program codenamed Milkwhite, giving them information about suspects that they either wouldn't have or would need to collect themselves.
While this was more of a warning than a definitive conclusion, the Intercept notes that there have been some real-world examples of mass surveillance failures leading to deaths. The extremists who murdered British soldier Lee Rigby in London were already known to MI5, but officials missed call and messaging records that would have revealed the killers' intentions. Like it or not, governments may need to worry less about expanding their powers and more about reining themselves in -- targeted spying could actually provide more useful intelligence.