It's an obvious educational simplification, but I don't remember our faculty ever getting much beyond the basic model. In the real world, of course, adult bodies are pretty entropic. There are lots of little things that don't work quite right, even in a healthy person. Add in age, chronic conditions, multiple simultaneous viral infections (having a cold doesn't make you immune to another!) and more and it's no surprise that diagnoses don't follow textbook traditions. Geriatrics is all about this kind of multi-factorial balancing act.
Turns out, the world of computing is no different.
Even as a reformed gerserker I still have to deal with the usual bewildering array of emergent bugs (emergent bugs are one reason many underestimate the power of the GooBook to come). These are particularly common in geriatric computing systems.
For example, until recently I had a 6 year old access point/router, a 6+ yo XP box, a 5-6 yo iBook, a 3-4 yo iMac, a 1yo iPhone and a 2 yo MacBook. When I started having network problems a few months ago I thought I had one thing to fix. Turns out it was about 3-4 things! In retrospect our home system suffered multiple interacting hardware and software problems with evolving features.
A new Airport Extreme/Time Capsule solved a bunch (after adding about 2-3 manageable new bugs), some software twiddling solved others, and, most recently, I think I might have solved another mysterious bug by upgrading my EMC Retrospect iMac client software.
Geriatric computing turns out to be a lot like geriatric medicine -- it's not one thing, it's everything.