Now there's the catch - "All circumstances"

Especially with mass-market software for general-pupose computers, it is (virtually) impossible to forsee all circumstances under which it might possibly be used.

eg, at the moment, I have over a dozen different applications all running simulateously on this same PC.
If you think about it, it's actually quite amazing that they do all manage to get along together, sharing the same RAM, the same CPU, the same disk, the same display, etc, etc, etc without interfering with each other!

Just look at the problems we see here with people managing to keep one interrupt from interfering with one other "task"...!!