A network loop, as the 3Com home connect dual speed hub manual constantly reminds me, is a network configuration where there is more than one path between two computers or devices, which causes packets to be constantly repeated. This is due to the fact that a hub will blindly transmit everything it receives to all connections - other devices, such as switches and routers, might be able to reduce or eliminate this problem.

In theory, this should be possible to create with only 2 hubs: For each hub, connect one end of a regular cat 5 cable in the uplink port and the other end in any other port of the other hub. (You could also use crossover cable between two non-uplink ports). Then connect at least one computer to each hub, and you have a network loop.