The fact if many network administrators don't realize how much they don't know about their network. The tendency is to think of a network as a static thing. Once it's built, it doesn't change much until the next major upgrade.
But the reality is that networks are dynamic and constantly changing entities. New software can change the entire way that data flows over the network. New devices, including switches and WiFi access points, can make their way onto networks without the administrator's knowledge. And security and access controls may not be to the level that you think they are.