Speaking with a business & commercial chap today he had a bit of a moan to me about architects worrying about edge scenarios. His complaint was that they kept trying to consider all of the edge conditions and then design around them when in fact he
a) didn't think those things would happen
b) even if they did happen he'd be fine for the system to collapse
c) thought the investment the architects had made in edge conditions was a waste of time
The point here is that sometimes its okay for a system to fail and also its okay to specify what a system won't handle rather than trying to make it handle everything.
Now some architects may leap forward and say "well what about the future, what about extension"
But you know what? If the business doesn't want to pay for these edge conditions then why on earth are you bothering? Sure you should force the point and say "the system won't handle X,Y,Z or pigs learning to type in ancient greek, is that okay" rather than "The system doesn't handle X,Y,Z or greek typing pigs so what we should do is redesign the keyboards to enable pigs to work with the system when they learn to type".
The sort of architectural "perfectionism" that underpins this mentality is, IMO, one of the worst traits of architecture, namely the avoidance of getting into the solution. By putting these edge cases in the way the architecture and design phase takes longer and the dirty solutioning piece when the architecture actually has to be proven can be postponed.
Edge conditions can be non-functional as well "What if your carbon blade procurement system is turned into a Facebook app and gets 10m hits in an hour?" The answer is of course "That won't happen, and even if it did we sell carbon blades to the airline industry so why on earth would we worry about a Facebook app?". Still architects will argue about the "most scalable" way to build the solution that is currently targeted at 5 end-customers with up to 20 concurrent users max.
Sometimes architects claim this is them being "professional" and considering the "future" while taking a "holistic" view. What rubbish, its about architects focusing on architecture and losing track of the big, and simple, picture and merrily pursuing their own mental exercises to demonstrate their architectural prowess.
Edge conditions can be hard to deal with, but that doesn't mean you HAVE to deal with them. Often, in fact most times, the right approach is to declare them out of scope and make it clear what the system won't do as much as what it will.