You might have hit a MAC address conflict as MAC addresses are not reserved by default: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.networking.doc%2FGUID-1B6A280E-0C77-4775-8F84-4B3F40673178.html.
Another thing to check is your vCenter Server instance ID: http://kb.vmware.com/kb/1024025.