Hello.. I noticed and thought I would post to save any other people time!
If you have Etherchannel configured as mode on on your core and you drop one of your edge devices out of etherchannel you easily create a loop.
Without PAgP or LACP configured both the Po and the physical link go into FWD state.. this is also true if you drop all your physicals out of the etherchannel.
The only way around this I can find is to NOT use [mode on] on your core... in fact don't use it anywhere.
Anyone know why it's there? Non-Cisco non-LACP perhaps??