Hey Patrick -
With portfast enabled, a port goes directly to forwarding mode and bypasses the initial STP steps. When go directly to fowarding *and* create a physical loop, you create an processor issue. This is due to L2 frames having no TTL field, thus continuing to loop infinitum. To stop the packets from looping, one of the following must happen...
1. Utilize STP to block one of the redundant ports.
2. Unplug one of the cables.
3. Wait for the sun to nova.
I would go with options one or two....
From a proactive approach, you do have BPDU guard enabled which is a great start. The mistake you made was not heeding the BPDU condition. If a port goes err-disabled, you should check to ensure a physical loop has not been created prior to re-enabling the port.
Your other option is to turn portfast off on the port, thereby allowing STP to do take care of blocking any secondary cable plugged in by accident.
HTH
