I have found already in two situations really problems when are configured connections between two Switches in Channel on fiber Gbic (trunk) and redundant Channel on copper FastEthernet(ever on trunk) from both Catalyst 6000 or 5000.
The connection became unstable and it seems that STP doesn't work in order.
In fact seems that the links on copper doesn't go in blocking but go in forwarding also it isn't configure with best priority naturally.
Do somebody know problems about STP with channel?
best regards
Lorenzo