I remember something vaguely like this problem. At work a rouge admin kept assigning ip addresses to machines w/o verifying he was entitled to them. He thought he owned a particular block.

Do you know that something needing an ip wasn't added or changed by one of the clueless.

Another incident had to do with the number of cables that were attached at the switch but not the far end. Remove any cable not actively in use.

Also, some switch/routers have a wan or uplink port. I have a router whose instructions prohibit using the wan port if all I want, is to use it for is extending the network.