Abstract: Proper orthogonal decomposition can be used to determine the dominant coherent structures present within a turbulent flow. In many flows, these structures are well represented by only a few high-energy modes. However, additional modes with clear spatial structure, but low-energy contribution can often be present in the proper orthogonal decomposition analysis, even for flows with a high degree of periodicity. One such mode has been observed in both free and impinging jets determined from particle image velocimetry. Both experimental and synthetic data are used to investigate the role of this particular mode, linking its existence to the unsteadiness of shear-layer large-scale coherent structures. Graphical abstract: [Figure not available: see fulltext.].