More IIUC notes.  As always, feel free to correct as needed.

To match inbound calls to a dial peer, CME (and CUCM?) uses the following steps.

1. Match DNIS (the dialed number) with the incoming called-address config in the dial peer
2. Match the ANI (the calling number or caller ID) with the answer-address config in the dial peer
3. Match the ANI with the destination-pattern config in the dial peer
4. Match an incoming POTS call to the port config in the dial peer
5. Match dial peer 0

Matching dial peer 0 is bad, and it took me an inquiry on Twitter and a buddy to realize why.  Here are a few highlights as to why.  I believe the full scope of the badness of dial peer 0 is really beyond the IIUC exam.