LAN Migration Done

Last week was busy migrating a site in Austria and preparing it so that it is ready for this week for the WAN migration. Site wasnt that big but configuring 1 x 6500 swtich, 22 x 3560 switches and migrating the users from the OLD entrasys switch in two days was tiring. But i am glad all of that worked without much of problems. Only one small problem which nearly brought the whole site down. I stil dont understand the real problem but if someone can shed some light on why this happened then that will be great.

What happened was that i had configured port channel on port 49 and 50 . Ports 51-52 were empty. By mistake the Fibre SFP was connected on port 51. We never noticed that while racking the switch and connected the fibre on that. We kept on racking the switches but after around 10-15 minutes users started complaining that they are not getting any IP from the DHCP and within 10 more minutes everyone in the office was complaining. I was quick to find the problem as i worked my way backwards and within 5-7 minutes i traced back to the switch where the SFP was plugged in wrong. Plugged it back into the righ port and .......it worked.

I still dont know how can SFP plugged into a wrong port can cause loop and bring the whole network down. Your comments !!!!!