We document how we diagnosed data fabrication in the National Income Dynamics Study. Since the fabrication was detected while fieldwork was still on-going, the relevant interviews were re-conducted and the fabricated data were replaced with authentic data. To the best of our knowledge, this is the first time that this has been done. We thus have an observed counterfactual that allows us to measure how problematic such fabrication would have been, had it remained undetected. We implement a number of estimators using the data that include the fabricated interviews, and compare these with the corresponding estimates that include the corrected data instead. For the outcomes that we consider, we find that the fabrication would not have substantially affected our univariate estimates. However, the fabricated data do impact substantially on some key covariates when panel estimators are used.