Update: This challenge is closed, see the final results (November 4th 2008).
Assuming you have the following configurations on R1 and R2:
What IP address can you use on the loopback interface of R1 to establish adjacency between R1 and R2? Can you use more than one IP address?
This challenge was triggered by a comment uri wrote on the “OSPF ignores subnet mask mismatch on point-to-point links” post, claiming that you cannot mix numbered and unnumbered interfaces in OSPF.