Title: Catastrophic overfitting is a bug but it is caused by features. Abstract: Adversarial training (AT) is the de facto method to build robust neural networks, but it is computationally expensive. To overcome this, fast single-step attacks can be used, but doing so is prone to catastrophic overfitting (CO). This is when networks gain non-trivial robustness during the first stages of AT, but then reach a breaking point where they become vulnerable in just a few iterations. Although some works have succeeded at preventing CO, the different mechanisms that lead to this failure mode are still poorly understood. In this work, we study the onset of CO in single-step AT methods through controlled modifications of typical datasets of natural images. In particular, we show that CO can be induced when injecting the images with seemingly innocuous features that are very useful for non-robust classification but need to be combined with other features to obtain a robust classifier. This new perspective provides important insights into the mechanisms that lead to CO and improves our understanding of the general dynamics of adversarial training.