Sharing IP addresses on interfaces

Note how Cisco IOS will prevent you from configuring the same IP address on more than one interface. For example

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#int ethernet 0
Router(config-if)#ip address 10.10.10.1 255.255.255.0
Router(config-if)#no shut
Router(config-if)#int serial 0
Router(config-if)#ip address 10.10.10.1 255.255.255.0
% 10.10.10.0 overlaps with Ethernet0
Router(config-if)#

Now, given this fact can you tell me what legal configuration results in the following output that clearly shows the same IP address on two different interfaces?

Router# show ip interface brief

Interface    IP-Address      OK?    Method   Status   Protocol
Ethernet0    10.10.10.1      YES    manual   up       up
Serial0      10.10.10.1      YES    manual   up       up

There is enough information here to answer the question. If you need any more information or if you think you know the answer, use the comments for this blog entry to communicate that.