Lets suppose that S1 is hard coded to be an etherchannel and S2 doesn't have etherchannel config , how the loop will occur?
in case that:-
1-A device connected to S1 send a broadcast or unknown unicast , will the etherchannel follow the default load balancing algorithm and choose only 1 link inside the Port-channel to forward the frame? or in this case will use both?
2-once the broadcast frame goes to root , it will flood the frame out of all ports expect the port it received the frame , S2 will send back the frame from the other port to S1 , however S1 will detect it received the frame on "Po1" as a whole not fa0/1 or fa0/2 so its impossible to transfer the broadcast back to s2 , so how the loop will occur in case of etherchannel misconfig?
Check if this discussion can give some help: