I had my first "network loop" last week. Apparently this is a miserable problem for many a systems administrator, but it didn't happen to me, until now, anyway. Why? Well, we didn't have any wireless routers at our office until recently.
Someone came in and had a poor connection using wireless, so plugged in an ethernet wire. At that point, the laptop is connected to the network in two ways, forming a loop, along which packets of data traffic ran in circles and made our lovely little network into one big traffic jam.
I have told my notebook users that they must use only one connection - wireless or wired, but not two!