An economic indicator, such as gross domestic product (GDP) or the genuine progress indicator (GPI), is a number that quantifies a particular aspect of an economy. Indicators are useful for comparing different economies or for monitoring development. But they are overly simplistic in that they ignore all non-quantifiable aspects of living. This flaw becomes crucial when an indicator is turned into an optimization target. Politicians will quickly identify and exploit mechanisms that are likely to increase the indicator, even if there is no benefit for society. Measures that would not even be considered in the absence of a specific optimization target can then become political priorities when that target is adopted. Public debt is one such indicator that has recently become a high-priority optimization target in many European countries, despite wide recognition of the socially negative effects of the cuts that are needed to reduce it. This will happen to any indicator, including the GPI, which takes into account social and environmental factors as well as economic ones. Economists and politicians must accept that no single number can safely be optimized. Several indicators that concentrate on different aspects of society need to be used in parallel, and any measure that improves one while decreasing another must be recognized as a compromise between conflicting goals.