The migration of mobile ions has long been considered a source of performance degradation in devices based on halide perovskites, but details regarding the mechanisms and extent of this problem remain scarce. Here we report the finding that differences in electrical potential across the plane of the substrate, induced by electrode edges such as the boundaries of the metallization or the transparent conductive oxide, can play a primary role in behaviour and degradation across a wide range of stability tests. In particular, we observe clear signatures of lateral ion migration in solar cells made using both double and triple cation mixed-halide perovskites, with effects induced by reverse-bias stress, illuminated MPP-tracking, and even as a result of time spent in storage. In their mildest form these effects manifest as patterns of modulated photoluminescence intensity around device boundaries that propagate into surrounding regions as a function of applied bias and time. We show that ionic drift-diffusion models provide a convincing match to these patterns, including their voltage dependence. In the context of MPP-tracking, we observe a severe form of material degradation in our solar cells whose spatial distribution and voltage dependence match models of lateral ion migration with high accuracy. We thereby obtain evidence of an ionically-mediated degradation mode which proceeds at a high rate near device boundaries, but is nonetheless also operative throughout the active area. This connection between "bulk" and "boundary" degradation has multiple implications for both the design of perovskite solar cell and LED devices, as well as for the understanding of their characteristic behaviour in long-term stability testing.