> I think this is just one example of how little we understand nature's systems.

Oh, definitely. And it applies at a very broad sense of the word "nature." For instance, weight-loss. As a physics guy I have a minor aneurysm whenever someone, referencing the obesity epidemic, says something like "a calorie is not a calorie". What they're trying to say is very important, but I dislike the way that it's expressed, and it may be the reason why it's hard to get those views more accepted.

What I'd say is: the physics of weight loss is extremely well understood, but the psychology of weight loss is not at all. I can tell you about thermodynamics but that doesn't really tell you, at the level you want, why everyone and everything is becoming obese. For a personal example, once I came from the Netherlands to the US Midwest, I rapidly gained a lot of weight, going from "skinny for my height" to having trouble with some Men's X-Large T-shirts. Is it because the portion sizes are in general much larger? Or is it because there's more fat or sugar or chemical additives here? Or is it because I'm not bicycling to work every day because distances are greater here? Or is it because in the Netherlands I was expected to "have a sport" (in my case ultimate frisbee) and I'm no longer able to maintain that time commitment in my new phase of life? Or is it because of a shift in my religious and personal priorities which made it harder to fast once or twice a week? Hey, maybe it's none of these! Maybe it's just that I'm now in a relationship with a teacher and her late-night grading causes me to get less sleep, but because I work a programming job and solve cognitively-complex problems, I have to spike my blood sugar constantly to maintain the same functioning. But maybe it's something else, like the fact that satiety signals only get sent from the stomach to the brain after 20 minutes, and here I just tend to eat more in that 20 minutes than there.

All I can really tell you as a physicist is that my body will naturally find some equilibrium where energy in = energy out, at which point weight will become mostly stationary -- and I can in theory point to the idea that I can change this by changing the situations that I'm in. In addition with a little bit of knowledge about hormones like leptin & ghrelin, I can tell you that when my fat cells eventually start to die, their equilibrium will signal hunger to my brain, making it hard to work off this weight without a lot of discipline about my food urges. (Another potential cause! I now have a refrigerator so food is "always around".) But even the littlest things, perhaps even getting an hour more sleep every night, could throw that whole psychological set of triggers completely in a different direction.

Some things, especially seen in the outside world, just have ridiculously complicated causal graphs. It's true of weather, ecosystems, biologies -- and even when those graphs are generated by some really simple rules like energy-exchange, their interactions can easily spiral out of control for how we see them.

Software is the same way. If each line has a chance of interacting with one other, that's going to be an O(n^2) causal complexity created by O(n) lines; good modules and interfaces only reduce the multiplicative constants but don't change the scaling. When you get to the 50+ million lines of Windows it's hardly remarkable that it has some bugs in it.