Think of it like iBGP without synch, and without full routing running in the cloud. Okay, that probably didn't make it any simpler. :-) Let's try this:
A----B----C
If B is marked as a "stub," as in a stub area, it should only have one exit. Suppose C knows of some path through A, but B doesn't know about it (or it only pays attention to it's default exit point, as a good stub should do). Now, C forwards a packet to B, and B happens to be using C as it's default exit point--a loop.
In reality, the problem is more fundamental, though--stub means "there's only one exit point from this area," while virtual link means "there's a path through this area." They are in logical conflict with one another just in defining them.
:-)
Russ.W
