This chapter disaggregates the mistakes, errors and miscalculations around counter-terrorism policy. We note the difficulties in assessing when ‘something goes wrong’ in counter-terrorism which centre around the issue of uncertainty. We reflect on how to calculate the impacts of counter-terrorism in objective and subjective terms, before considering specific aspects of counter-terrorism in examples from British counter-terrorism policing and measures. We distinguish three types of ‘mistakes’: ‘genuine’ errors, misapplication of policy and unintended consequences. We consider the effects of such mistakes, in the form of ‘suspect communities’ and the ways in which high-profile mistakes come to shape perceptions of counter-terrorism practices. Given the inherent uncertainty and the seeming decision to prioritise precautionary logics, ‘mistakes’ in counter-terrorism are inevitable
