It is not just manually flying where this is a threat. If descending using “FPA-Flight path angle” or “VS-Vertical Speed” modes to say make a level restriction, all can be fine when descending with a constant Mach number and there may even be a small amount of thrust still applied. But at transition the AP keeping that constant angle can cause the IAS to start running away and an over speed can easily happen if they crew are not effectively monitoring.