Looping back a cable to an ethernet hub will simply cause the ports in question to be disabled. I've never seen one that didn't do that. krogoth is thinking of a routing loop, and cannot be caused by hubs, only routers or possibly switchs. How this might happen is if some routing tables get screwed up so router a thinks the next hop is router b, which thinks the next hop is router a which thinks the next hop is router a, and so on, untill the time to live is decremented to zero.