A group of unsteady flow phenomena in turbomachinery is often referred to as “rotating instabilities.” They occur at flow conditions with very asymmetric velocity profiles, as found downstream of blade rows in axial compressors with large rotor tip clearance in stable operating points close to compressor stall or off-design operating points in steam turbines. In the present work, it is shown that such instabilities do not depend on the blading but also occur in blading-free flow channels. The theoretical proof is carried out with the tools and computational programs of spatial stability theory and Reynolds number dependent jumps in the transverse wavelengths at characteristic perturbation frequencies of the present asymmetric turbulent channel flow are discovered. Experimental investigations in a straight flow channel confirm the calculated frequency field and other characteristics of the perturbations. In agreement to theoretical results, a very asymmetric velocity profile across the channel height was found to trigger disturbances within a limited range of frequencies and not bound to the existence of blading. The notion of rotating instability is misleading in the characterization of this disturbance.