In this article we present three robust instability mechanisms for linear and nonlinear inverse problems. All of these are based on strong compression properties (in the sense of singular value or entropy number bounds) which we deduce through either strong global smoothing, only weak global smoothing or microlocal smoothing for the corresponding forward operators, respectively. As applications we for instance present new instability arguments for unique continuation, for the backward heat equation and for linear and nonlinear Calderón type problems in general geometries, possibly in the presence of rough coefficients. Our instability mechanisms could also be of interest in the context of control theory, providing estimates on the cost of (approximate) controllability in rather general settings. Contents 1. Introduction 1 2. Preliminaries 8 3. Abstract framework for instability 16 4. Instability in the presence of global smoothing 30 5. Instability at low regularity 34 6. Instability in the presence of microlocal smoothing 52 Appendix A. Instability for linear inverse problems 65 Appendix B. Proofs of some results in Section 2 73 Appendix C. Carleman estimates and Hölder instability of interior UCP 77 Appendix D. Gevrey wave front sets and oscillatory integrals 81 References 84