We report on the observation of structures in the longitudinal resistance of a single two-dimensional electron gas (2DEG) in a perpendicular magnetic field at filling factor νcongruent with1±0.1. The observed structures can be switched off by depleting a second 2DEG, which is separated from the first by a 30 nm or a 60 nm AlGaAs-barrier, using a gate-electrode. Measurements as a function of temperature and drive current indicate that these structures are precursors of the breakdown of the quantum-Hall-effect (QHE) at ν=1. The QHE becomes unstable when another 2DEG is in the neighbourhood. This effect is most likely due to macroscopic effects like screening, because other coupling processes between these two 2DEGs, such as frictional drag, are weak.