Most EU member states adopt national fiscal rules that refer to cyclically adjusted borrowing limits. Yet, trend increases in public debt caused by the cyclical components are only prevented if the real-time output gaps used to calculate cyclical components balance over time. We analyze real-time output gaps for EU-15 countries estimated by the EU (2002–2012), the IMF (2000–2012), and the OECD (1989–2012) and find a strong negative bias. Simulations suggest that this bias may well translate into considerable debt-ratio hikes. Our findings imply that fiscal rules should incorporate ex post checks of the unbiasedness of cyclical components and a corresponding correction mechanism.