I had a similar issue in which someone had accidentally looked a Cisco VoIP phone back into the network. However, I found it strange how often this would occur and eventually came across this field notice that might apply to others on the list:
http://www.cisco.com/en/US/ts/fn/610/fn61863.html Problem Description Disconnecting power from a locally powered Cisco IP Phone connected to a non-Power Over Ethernet (POE) Cisco switch may expose the customer's network to loop back storms that destabalize the virtual local area network (VLAN). This exposure can be mitigated by configuring the switches with automatic loop detection and port recovery. Notes indicate this normally applies onto to 10Mb connections, but that there have been reported cases on a 100Mbit network when the VoIP phone is connected to a "highly sensitive uplink switch". Trey
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