OSPF Path Selection

So this has actually been bothering me now for YEARS. In the CCIE RS Exam
Certification Guide, there is a paragraph that goes something like this:

*OSPF has specific rules for selecting a path that crosses areas. *

*1) Take the shortest path to area 0.
2) Take the shortest path across area 0 without traversing a nonzero area.
3) Take the shortest path to the destination without traversing area 0.*

This has always been somewhat vague and even disturbing to me. It's
seemingly vague and no other explanation is given about this process. Rule
1, take the shortest path to area 0 makes sense. Once you get to the
backbone area, rule #2 even makes sense. But rule #3 has never and does not
make sense to me

So far as I recall, an OSPF ABR will never accept type 3 summary LSA
information from a non-backbone area. In other words, If an ABR receives
inter-area routing information for a non-backbone area from a non-backbone
area it is ignored. This makes sure that inter area routing information is
only learned from the backbone area, and is also a loop prevention
mechanism. Further, in my mind it guarantees that all inter-area traffic
must transit the backbone.

With that being said, can anybody think of ANY case EVER where rule #3 is
even valid? How would it ever be possible for inter-area traffic to get to
a destination without traversing area 0?

-- 
Regards,
Joe Astorino
CCIE #24347
http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
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