Symmetric sand ripples formed by water waves are common features on modern coasts and in sedimentary rocks. The size and spacing of wave ripples generally scale with water depth and wave conditions, and are widely used to reconstruct coastal environments of the geologic past. Interpretations based on average ripple dimensions and assumed constant wave conditions are informative, but many rippled beds contain striking patterns involving defects—deviations from straight, evenly spaced ripple crests—that suggest more dynamic flow regimes. We report a set of laboratory experiments that reveal how these patterns form in rippled beds adjusting to a change in wave conditions. As the ripples in our experiments evolved toward a new spacing, they developed defects that are widely observed in modern environments and in the rock record. The dominant defect type depends on the sign and magnitude of the adjustment in ripple spacing and the number of wave periods since the change in wave conditions. A regime diagram summarizing these associations quantitatively links ripple defects to transient flow conditions. Our experiments reveal the origin of previously unexplained ripple patterns and add a new dimension to paleoenvironmental interpretations.