I just got off a 30 minute chat with Cisco where we were trying to figure out why I couldn't access the web interface on my E3000 router when it was connected to the rest of my home network. The whole conversation was a little annoying because I'm supposed to know what I'm doing.
It turns out that the underlying problem was a new DirecTV box my husband hooked up a couple of weeks ago. If you're tech-savvy, you know that most if not all of the network-enabled devices you buy these days are shipped with a static IP address of 192.168.1.1. So I imagine there are a lot of people out there who get really frustrated when they hook up that new router or DirecTV to a network that already has a device at that address and all hell breaks loose.
I didn't have a clue what the problem was until I had one laptop connected to the E3000, the E3000 disconnected from the rest of the network and a second laptop connected to my home network with the wireless disabled. I pinged 192.168.1.1 and it still answered -- something that could only be possible if there were another device with the same IP address.