I have two 8 port hubs (One 100mbps and one 10mbps) which are connected. For some reason yesterday right after putting XP Pro on a spare machine and joining it to my domain the XP machince couldn't see any machines sonnected to the main hub. Naturally it's connected to the secondary one. The other two machines aren't on at the moment as I've kinda run out of powercables, so I couldn't really test the connection with either of those two.
At first I thought it was a routing or TCP/IP issue and looked to see if I could figure it out. Nothing. It all looked good.
Today my housemate came to tell me that he couldn't connect to the internet anymore. I had unplugged him from my main hub, as I wanted a port there for myself, and plugged him into the secondary one. So I went to plug the cable into the other hub (He does pay me for the net access after all...) and lo and behold, the cable connecting the two hubs had fallen out...